sounds like its stuck in DFU Mode
If iTunes doesn’t see your iPhone’s cry for help, you may need to invoke recovery mode. Although it’s an extremely helpful mode, how to get there isn’t very apparent. Read on for the how-to!
As arcane as “recovery mode” sounds, it’s actually very simple:
Disconnect your iPhone and turn it off by holding the “Sleep/Wake” button for about five seconds and then sliding the red slider. If your iPhone is already off, there’s no need to turn it on and then off again. Once it’s off, press and hold the “Home” button and plug your iPhone into a Mac or PC with iTunes installed. Keep holding the “Home” button until you see a dock cable pointing to the iTunes icon. iPhones running older software will display a yellow triangle and text which says “Please Connect to iTunes”. Once you see this image, you can release the Home button.
Open iTunes (if it’s not already open) and you will be told that “iTunes has detected an iPhone in recovery mode.” Click “OK” and you will be taken to the iPhone pane. Click “Restore” and you’re done, it’s that easy! Depending on the extent of your iPhone’s ills, it may take some time to do a full restore. Once you’re done, you should again have a perfectly working iPhone.
If this doesn’t solve your problem, you may need to restore using DFU (Direct Firmware Upgrade) mode. To enter this
mode, plug in your iPhone and turn it off. Hold down the “Home” and “Sleep/Wake” buttons for about twelve seconds. You’ll briefly see the silver Apple logo and then your phone will appear to turn back off. Release the “Sleep/Wake” button but continue to hold the “Home” button until you see a message in iTunes stating that it has detected an iPhone in recovery mode. Choose “Restore” and you should be fixed right up!
If this doesn’t fix your problem, your iPhone may require a visit to the Genius Bar