have you checked via Nokia Suite if there are any firmware updates available for your phone? if there are any, please update to the latest version offered. failing that, you can try re-installing your current firmware. if you are still having problems, then try performing a three-finger hard reset....make a backup of all personal data to PC and/or online storage first, as reset wipes everything.
Failign the above, you will need to take your phone to your local Nokia Care for inspection:
http://nokia.com/support