If your iPhone hangs completely you may need to give it a hard reset, the iPhone equivalent of Ctrl+Alt+Delete. To reset the iPhone, hold down the sleep/wake button on the top of the phone and then press the Home button. After a couple seconds of this, the Apple logo will show up on the screen. You can then release both buttons and wait for your iPhone to come back up.

Only do this if you are *really* stuck since resetting your iPhone in this way can result in data lose if you were in the middle of doing something.