Try restarting it (turn it off and back on). Now if you did the one that allows you to save your personal data (the reset) then this may be your problem. In such a case, I suggest you do a "Hard Reset", instructions below, and this may resolve the issue.
NOTE: Doing this will clear 'all' the data on your phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the device.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the phone...
1. Make sure that the phone is actually powered off.
2. Remove your SD card, if there is one in the device.
3. Press the "Power" and the "Volume Up" buttons, together...