hi mate,
have you checked if there are any firmware updates available for your device via Nokia Suite on your PC? if there are any, please update to the latest version offered. Failing that, you can try re-installing your current firmware thru Nokia Suite....if that doesn't work either, then try Restoring your phone to factory settings.
Failing the above, you will need to take your phone to your local Nokia Care for inspection:
http://nokia.com/support