Restore in DFU Mode.
Connect iPhone to iTunes and Restore Firmware in DFU Mode. To get DFU Mode, with iPhone connected and iTunes open, hold both Home and Power buttons for exactly 10 seconds and then release Power button, keep holding Home button until iTunes recognizes iPhone in Recovery Mode about 20 seconds time. If the iPhone screen is black then this is DFU Mode. Restore the iPhone. If the screen is not black, keep trying the steps until you get DFU Mode.