You may need to try a factory reset, but it`ll erase all your data, so be careful:
1) Make sure phone is turned OFF
2) Hold both the Volume down button and the Power button
3) Volume down key to scroll to "Factory reset"
4) Press the Power key to start reset
5) Press the Volume down key to scroll to Yes - delete all user data
6) Press the Power key
7) When "Reboot system" now highlighted, press the Power key