How can I force my iPhone to reboot / restart / reset?
Sometimes your iPhone can lock up completely and not respond to any input. In this case, the iPhone can usually be forced to reboot:
1. Hold down the sleep/wake button and home button simultaneously.
2. Let go when the Apple logo appears and the iPhone is booting up.
This will usually help, however if the iPhone fails to boot you may just see a blank black screen. In this case you might need to restore the iPhone using your computer and iTunes. Follow these instructions to restore your iPhone.