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.
If the device won't power on, press and hold the Power, Volume Up and Volume Down buttons for 1-2 minutes.
Press and hold the Power button (located on the top edge) until the Phone options screen appears then release.
From the Phone options screen, tap Restart.
From the Restart phone? prompt, tap Restart. Allow several seconds for the device to power off.
Press and hold the Volume Up button (located on the right edge) & the Power button (located on the top edge) until the device powers off (approximately 10 seconds) then release. Allow several seconds for the reboot process to complete.