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.
Won't affect data on the device.
Press and hold the Power button (located on the back) until the Phone options screen appears then release.
If the device is unresponsive, press and hold the Power button until the device powers off (approximately 10 seconds) then release.
From the Phone options screen, tap Power off and restart.
From the Power off and restart prompt, tap OK to confirm.
Allow several minutes for the process to complete.
If unable to perform a soft reset, remove then re-insert the battery.