iPhone won't turn on? Try resetting

If your iPhone suddenly will not turn on despite being fully charged, try resetting it by pressing and hold the Sleep/Wake button (on the top right of the unit) and the Home button (circular button on the iPhone's face) simultaneously for at least 10 seconds, until the Apple logo appears.

Failing that, try restoring the phone by connecting it to your Mac or PC and, in iTunes, clicking the Restore button. Restoring the phone will erase contacts, calendars, photos and other data on the phone, but will restore automatically backed-up information including text messages, notes, call history, contact favorites, sound settings, widget settings, etc.