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 (located on the top right edge) until "slide to power off" appears then release.
Alternative method: Settings > General > Shut Down > slide the Power switch right.
If the phone is off or not responding, refer to soft reset for unresponsive iPhone.
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.