Hi. Try a different USB port on the PC or even a different PC if you have no luck with GD Solutions answer.
If the phone was disassembled, go back and check your Dock to logic board connector is seated properly. I.e, disconnect it and re-attach it. You can try it with the PC again without having to put the back cover screws back in. If still no joy, another thing to try is putting the phone into DFU mode.
In DFU mode the screen is blank and the device appears to be off, but iTunes detects it as an "iBoot recovery device" or similar and it can be restored in iTunes.
To get into DFU mode:
1) Connect the USB cable with the device powered on.
2) Hold Home+Power until the device resets (about 10 seconds)
3) As soon as the screen goes blank, release Power and continue to hold Home.
You have to be really quick about releasing power. If you did it right, the device should appear to be off (no Apple logo appears), but iTunes will ask you to restore.