Perform a soft reset. If that won't help fix the issue, you might need to do a hard reset. If that still won't work, you may need to send your phone for repair.

Soft Reset:
- Hold the volume down + power button pressed until the phone vibrates and restarts

Hard Reset:
- Switch off your phone
- Press and hold Camera + Volume down + Power buttons until your phone vibrates
- Once your phone starts vibrating, release power button but keep Camera + Volume down button pressed for more 5 seconds