Apple iPhone hard reset
If iPhone won’t turn on, or if the display freezes or doesn’t respond
Press and hold the Home button below the screen for at least six seconds, until the application you were using quits.
If that doesn’t work, turn iPhone off and turn it on again. Press and hold the Sleep/ Wake button on top of iPhone for a few seconds until a red slider appears, and then drag the slider. Then press and hold the Sleep/Wake button until the Apple logo appears.
If that doesn’t work, reset iPhone. Press and hold both the Sleep/Wake button and the Home button for at least ten seconds, until the Apple logo appears.