My macbook air wont start

http://support.apple.com/kb/ts1440

A flashing question mark appears when you start your Mac Symptoms

If you see a flashing question mark when you start your Mac, it's probably because it can't find the system software it needs to start up. Usually, all you have to do to get your Mac back up and running is remind it where its software is.
Resolution

If your computer starts up normally after a brief delay, you probably just need to reselect the startup disk in Startup Disk preferences. It's normal to see the flashing question mark when a startup disk has not been selected. In most cases, reselecting the startup disk is all that is required to resolve the issue.

Sometimes, your computer may not start beyond the flashing question mark.

Tip: If your computer has a gray screen (with no flashing question mark) startup issue, see Mac OS X: Gray screen appears during startup.

Check the mouse and keyboard

This issue might occur if a mouse or trackpad button is pressed during startup. Make sure the button isn't pressed.

If the issue persists, shut down your Mac with its power button, disconnect any external mouse and keyboard, then turn on your Mac with its power button. If the flashing question mark issue persists, reconnect the keyboard and mouse.

Additional steps

If your Mac still starts to a flashing question mark, follow the steps below. If any step resolves the issue, you don't need to continue to the next one.

1. Select your Mac OS X startup disk with Startup Manager by restarting and holding the Option key. After your Mac starts up, restart again to verify that the flashing question mark does not appear.
2. If the issue persists, insert your Mac OS X installation disc. Be sure to either use the disc that came with your Mac, or, if you installed a later Mac OS X version from disc, use the newer disc.
   

   MacBook Air note: On a MacBook Air, there are two options for starting up from Mac OS X media: Either connect a MacBook Air SuperDrive to the MacBook Air via the USB port and restart the computer, holding down the C key during startup, or use Remote Install Mac OS X to startup from a system software DVD that's located on a partner computer. Once started up from Mac OS X media, skip to step 3.
   
   

3. Restart the computer, then hold the C key during startup.
4. From the Utilities menu, choose Disk Utility. Don't click Continue.
5. Select your Mac OS X disk (named "Macintosh HD" by default) in the left side of the Disk Utility window.
6. Click the First Aid tab.
7. Click Repair Disk to verify and repair any issues with your Mac OS X startup disk.
8. After repairing the disk, try to start up normally.
   
   Important: If Disk Utility finds issues it cannot repair, you may need to back up as much of your data as possible (or use Time Machine to back up to a different disk), then erase the disk and reinstall Mac OS X. You should back up important files and data before erasing a drive. Erasing deletes everything on the hard disk (including things on your desktop). Also, you can install Mac OS X onto an external disk, start from the external disk, and use Migration Assistant to transfer items from your usual internal Mac OS X startup disk to the external disk, then erase the internal disk and reinstall Mac OS X.
   
9. If the issue persists, and Disk Utility didn't find any irreparable issues, quit Disk Utility, quit the Installer, select your disk when prompted, and restart.
10. If the issue continues, reset PRAM. Note: After resetting PRAM, if the computer starts up normally, reselect the startup disk in the Startup Disk preferences.
11. If none of these steps resolve the issue, start up from the Mac OS X Installation disc and reinstall Mac OS X.

Last

Resetting the PRAM may do the trick:
You reset the PRAM by rebooting a Mac and then immediately hold down the Command+Option+P+R keys, you will then hear the Mac reboot sound again, signifying your PRAM has been reset. You must hit the key combination before the grey screen appears otherwise it won't work.

you've tried rebooting, you've reset the PRAM, you've done it all, but your Mac is still behaving strangely. What next? In certain circumstances, resetting your Mac's System Management Controller (SMC) can be a solution. This is sometimes necessary to restore normal lower level system functionality to your Mac.

Here's how to reset the SMC on any type of Mac (and any version of OS X) and the type of problems that it may resolve.
When & Why Reset SMC on a Mac? Some Common Hardware Reasons

Typically it's power and hardware related issues that can be helped or resolved by resetting a Mac SMC, particularly if you're having these types of problems:

* Issues with your Mac's cooling fans and fan management: fans run constantly at high speed, fans run high despite low CPU usage and adequate ventilation, fans not working at all, etc

* Power management and battery problems: Mac isn't turning on, sleep isn't working, random shutdowns and reboots, battery isn't charging, Mac won't wake from sleep etc

* Light problems and improper lighting management: battery indicator lights aren't working, display backlighting isn't adjusting to ambient light changes, keyboard backlights not working, etc

* Video and external displays not working: display brightness functionality not working properly, target video mode isn't working properly, external display isn't working, etc

* General performance and functionality problems: abnormally sluggish behavior despite no CPU or disk usage, external ports not working, airport & bluetooth aren't showing up, external devices are not being found, etc

So now that you've concluded you need to reset your SMC (Note that the SMC is on Intel Mac's only), here's how to do it on MacBook's, MacBook Pro's, iMac's, Mac Mini's, and Mac Pro's. Notice that the reset instructions are different for MacBook and MacBook Pro's if you have an internal or detachable battery.
Reset SMC of a MacBook Air, MacBook Pro Retina, or MacBook Pro's with an internal non-removable battery

This is how to reset SMC on most modern Mac laptops:

Shutdown your MacBook Air / MacBook Pro
Connect the power adapter to your Mac
On the MacBook/Pro's keyboard, hold down the Shift+Control+Option keys and the Power button at the same time
Release all keys and the power button at the same time
Boot your Mac as usual

Here's the key sequence to hold down:

Reset the SMC of a MacBook Air or Retina MacBook Pro

Do keep in mind that by resetting the SMC you will lose power specific settings, like the time it takes to sleep a Mac and other customizations to power settings. No big deal, but if you have made many changes to your hardware behavior you will want to adjust stuff like sleep behavior again.
Reset SMC of an iMac, Mac Pro, Mac Mini

Resetting SMC is different for non-portable Macs:

Shut down your Mac
Disconnect the power cord
Press and hold the Mac's power button for 5 seconds
Release the button
Reattach the power cables and boot the Mac as usual

Reset the SMC of a MacBook or MacBook Pro's with detachable batteries

mac reset the smc Older MacBook laptops with a removable battery can reset SMC with the following approach:

Shutdown the MacBook/Pro and remove the battery
Disconnect the power adapter, hold the Power Key for 10 seconds
Release the power key and reconnect your battery and power adapter
Turn your Mac on
Let boot as usual

Hopefully that will clear up your problems, if not it might be worth a visit to the Apple Store or a certified repair center.

Happy New Year,
Kerry

Have you looked at the article on https://discussions.apple.com/thread/3686085?start=0&tstart=0 at the bottom user shldr2thewheel has a solution that may help you. Hope it helps.

Try restarting your computer while holding down the X key. This may force the computer to start up using your Mac OS X system. If it does, release the X key, then open System Preferences and click Startup Disk. Make sure your Mac OS X system is selected. If you are unable to start up using the X key, restart your computer and immediately press and hold the Option key. Release the Option key when icons start to appear in the center of the screen. Select the Mac OS X system disk, then click the forward arrow to continue starting up.
If the above attempts to reselect your Mac OS X startup volume are unsuccessful, try resetting PRAM. Shut down your computer, then turn on the computer and immediately press and hold Command-Option-P-R until you hear the startup sound for the second time. Open System Preferences, click Startup Disk, and select your Mac OS X volume.
Hope this helps. If none of these options fix your problem, bring your computer to an Apple store, they are always very helpful and almost never fail to fix the problem!

Unplug it, remove the battery. Once it shuts down, replace the battery, plug it back in and start it up.

Hold the shift key down and start in safe mode.
Once the Macbook Air has restarted use the original install disk to repair the startup disk

Thank you for contacting Fixya Support.

Try these steps:

- - Reboot your Mac

- - Upon restarting your Mac, press the Shift key.

- - Hold the Shift key until you see a White Screen with Gray Apple icon and eventually a spinning gear.

- - When you see the Spinning gear, release the Shift key.

- - Allow the Mac to continue booting up. Also this may take a little while

(NOTE: this may take anywhere from 5 to 15 minutes to load to the Safe mode screen depending on the performance of your Mac, but if it exceeds this time, shutdown your Mac and try just once more. It may have just frozen up the first time around for you.)

- - You are going to enter the Safe Mode (which looks like your main desktop environment - the only difference is certain startup items are disabled.)

- - Once within the Desktop in Safe Mode

- - From the Finder Menu, click Go> Go to Folder

- - Type in this line: /System/Library/StartupItems

- - This path, is going to take you to the System Library folder which has all of your Mac OS X Startupitems.

- - Remove all of those folders to the Trash.

*Reboot your Mac normally and see if you can access your normal User desktop environment.

This should help.

Regards,

-Mactechtrainer

If you have your Mac OS X Install disc, insert it into the drive.
Restart the computer and hold down "C" while booting.
Choose your language.
Select "Disk Utility" from the "Utilities" in the menu bar.
Select your Macbook's hard drive in the left sidebar.
Click the "First Aid" tab thats along the top of the viewer window.
Click "Verify Disk" (not verify disk permissions)
If this shows errors click "Repair Disk"
Once completed close the Disk Utility window.
Restart the computer.
If this fails to fix it as well then your next option would most likely be recovering your files from the macbook either by booting into target disk mode (hold down T at startup) and transferring them via a firewire cable, or removing the hard drive and hooking it up to another Mac to get the files. Then do a clean install of Mac OS X using your installation discs.

When troubleshooting a Macbook Air, there is a checklist from Apple Support you need to try.
You are welcome to do so after first trying these steps first.

• Check the Magsafe power adapter, to make sure it is free of debris and is seated properly.

• If with decent charge, try to start the Macbook Air, with only the battery.

If you get power from either one of these issue resolved- otherwise follow the steps provide more in detail from Apple listed in the link below.
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Apple Support Link:
Troubleshooting Macbook Air (additional steps)
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Such power issues with Macbook Airs as well as Macbook 13" and Macbook Pro 13" are about the same and there are no workarounds, so make sure to follow those procedures completely for a full assessment of your Macbook Air's functionality.

Hope this helps!

Regards,

-Mactechtrainer

Most likely fix is to reset the SMC.
Turn the power off, remove all peripheral cables, connect the AC power supply. Using the left-side keys, press the combination Shift-Control-Option-powerbutton ONCE. The LED on the MagSafe connector
should go off, go back to green when keys are released, and may change to orange if the battery still needs some charge. Wait five seconds, then press the power button to restart the computer.