How to hard reset your frozen iPhone or iPhone 3G
If your iPhone iphone stopped working and doesn't respond or freezes try this steps:
1. Off -> On
First try 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 slide the slider. Then press and hold the Sleep/Wake button until the Apple logo appears.
2. Reset iPhone
If that doesn't work, reset the device: Press and hold the Sleep/Wake button and the Home button at the same time for at least ten seconds (ignore the "Slide to power off" slider if it appears and keep holding), until the Apple logo appears or the screen fade to black (the turn it on using Sleep/Wake button).