A 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. In most cases, a soft reset won't affect data on the device, unless the battery is very low. It is recommended the battery be charged for at least one hour if this is the case.
Alternate Method (use if device is unresponsive or has a frozen screen)
Press and hold the Power button (located on the back) until the device powers off (approximately 5 seconds) then release.
Allow several seconds for the reboot process to complete.