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have you checked via Nokia Suite if there are any firmware updates available for your device? IF there are, please update to the latest version offered. Failing this, try re-installing your current firmware, and if this doesn't work either, then you can try performing a three-finger hard reset. Please remember to backup all personal data on your PC or online storage equivalent.
Failing all of that, you will need to take your phone to your local Nokia Care point for them to inspect: http://nokia.com/support