Open iTunes on computer, connect iPhone to computer, hold both home and power buttons for about 20 seconds until iTunes recognizes iPhone in Recovery Mode. Click the Restore button of iTunes. If still problem, try
DFU Mode Restore: Open iTunes on computer, connect iPhone to computer. Hold both Home and Power Button for exactly 10 seconds, then release the Power button, keep holding the Home button until iTunes recognizes the iPhone in Recovery Mode, the iPhone screen should be black, blank. Click the Restore buttons of iTunes.
iTunes needs to be up to date version 10.7, if you need this, click the iTunes tab at the top of this window screen.