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A normal restart only requires you to press the re-start button on the back of the CPU or on the front. You should hear the chime, the screen will come up and you will be taken to the Mac start-up screen (different OS will have different looking screens).
If the Imac has shut down suddenly or prematurely, you might not hear the chime but will see a grey screen with a status bar. Eventually you will see the normal Mac screen and your icons will slowly reappear.
If you do not get any start-up screen or any chime, unplug the unit, remove all peripherals ( such as USB cables etc) and leave it sit overnight. I have restored "dead" Macs by that simple technique.
If you see a screen but the Imac still won't boot, insert the restore disc, hold down the "C" key and restart from the disk. Or use disk warrior and start the same way. Disk Warrior will diagnose and repair problems with your OS.
If none of these strategies work, you may have internal component problems with your Imac that will require more expert repairs.
A boot chime means the Mac has passed POST, that the screen remains black initially suggests there is a problem getting image to screen which could mean a bad cable to the LCD or the LCD display its self is having problems.
By connecting to an external display you verify the graphics on the logic board is functional, and it could get you up and running till the display problem is fixed.
Also try powering up and when you hear the chime, hold down the COMMAND+OPTION+P+R keys, and keep holding them till the machine has chimed 3 times, this will reset the 'BIOS' in case there is a glitch there stopping the display from showing up.
If the start up sound plays and the hard drive sounds normal try connecting the computer to an external monitor. If the external monitor shows the desktop just fine then the cable connecting the laptop to its screen is probably going bad.
If the computer will not boot at all try zapping the PRAM by
Pressing the power button and then command-option-p-r before the gray screen comes up.
Hold those keys down until you hear the startup chime.
Let go of the keys and let the Mac reboot.
If that doesn't work try booting from an install disk and run a repair utility.
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button. When the menubar appears select Terminal from the Utilities menu. Enter resetpassword at the prompt and press RETURN. Follow instructions in the dialog window that will appear.
Have you zapped the p-ram? Hold Apple, Option, P and R keys when you hear the chime. Wait for four more chimes and release.
Try "detect displays" from display preferences. In system preferences, displays, be sure brightness control is turned up.
When you press the power button do you hear a chime. If you do your monitor cable is probably loose. If you do not hear a chime there is a physical problem with the computer and you should make and appointment with the apple store to have it checked out.
It seems likely that the PMU (Power Management Unit) has failed and needs to be reset. There are comprehensive instructions on how to do this at the apple support site.
Apple Support: PMU reset
There is an elegant way to solve this, but it requires another compatible model Mac.
Boot the damaged Mac into FireWire Transfer mode by holding down the "T" key immediately after hearing the startup chime. Attach a FireWire cable between the two Macs. Start up the other Mac by holding down the "option" key after hearing the startup chime. Choose the startup drive from the broken Mac. Now your good Mac is booted off the drive of the broken Mac, and any configuration changes you make will be made on the hard drive of the broken Mac.
Attach the external monitor temporarily to the good Mac. Open System Preferences and set the monitors up in mirrored mode. Shut down the good Mac. Then hold the power button in on the broken Mac until the power turns off, and remove the FireWire cable.
Now move the external monitor to the broken Mac and boot it. It should come up in mirrored mode.
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