hi mate,
have you checked via Nokia Suite on PC if you have the latest firmware update installed on your device? if you do, try re-installing your current firmware thru Nokia Suite. if that doesn't work, then try performing a three-finger hard reset, but perform a backup of all personal data first to PC and/or online storage first.
Failing that, you will need to take your phone to your local Nokia Care for inspection:
http://nokia.com/support