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Have you checked if there are any firmware updates available for your device via Nokia Suite? If there is one, please update to the latest version offered. If there is none, then try re-installing your current firmware through Nokia Suite. If that does not work, then try Restoring your phone to factory settings.
If that doesn't work, then you will need to take your phone to your local Nokia Care point for them to inspect. to locate your nearest Nokia Care point: http://nokia.com/support