A restart (soft reset) can resolve issues like continuous resetting/rebooting, device crashing, freezing, won't ring or vibrate, can't make or receive calls, device locks up, and keypad doesn't respond.
A soft reset (turning the device off then back on) is similar to restarting a desktop computer and is one of the first steps in troubleshooting a device. A soft reset won't affect data on the device.
Press and hold the Power button (on the right edge) until "slide to power off" appears then release.
If the phone is off or not responding, press and hold the Power and Home buttons until the Apple logo appears (approximately 10 seconds) then release.
Slide the Power switch right.
Allow several seconds for the device to power off.
With the device powered off, press and hold the Power button until the Apple logo appears then release.