When the iPhone crashes or locks up in a way that prevents the iPhone from responding to the normal iPhone restart procedure, you need to reset the iPhone. Resetting the iPhone is simple.
Reset the iPhone by holding the sleep/wake button at the top right of the device and the home button at the bottom center of the face at the same time. Hold them both until you see the screen go black and the white Apple logo appear. When this happens, you can let go - the iPhone is rebooting.
Resetting the iPhone should not remove or delete any data or settings.
(Resetting the iPhone is also the term used sometimes to mean deleting all content from the iPhone and restoring from a backup.)
The reset process is the same on the iPhone 3G and iPhone 3GS.
