do a Soft Reset: By pressing down the Volume Down + Power buttons for 10-15 seconds (or until the device vibrates)
or if that doesn't solve it
Hard Reset: By going to settings>about>reset your phone you can completely wipe your phone of your personal data (ideal if you're going to send in your phone for repair or sell it). Your phone will turn off and you'll see spinning gears for a while (up to an hour or so) and when it's done you will see the same screens you saw when you first turned on your phone out of the box. You'll have to sign in with your Microsoft account again and start over.
Last Resort . Nokia Software Recovery tool