Y.I.N.E try a power cycle first "There are multiple ways to reset your device if you are experiencing issues. The first step is to power cycle your device. Press and hold the Power/Lock button, then select Power off. Select Power off to confirm. To turn the device back on, press and hold the Power/Lock button until the screen lights up.
Note: If you are unable to power your device off or if it becomes unresponsive, press and hold the Power/Lock button for 10 seconds, until it reboots." If that does not work check through other solutions for a software issue on here If none of those work, you have to dig a lot deeper into the bowels of your phone ;-) Follow this flow sheet to narrow it down. the images are best viewed when downloaded and using your own image viewer.