1. Press and hold the volume down key and power key at the same time for about 10 seconds. The phone restarts. To switch your phone back on, press and hold the power key.
2. Update your phone software.
3. Reset your phone. This restores the factory settings, and erases all your personal content, including apps you’ve purchased and downloaded. On the start screen, swipe left, and tap Settings > about > reset your phone.
If your issue remains unsolved, contact your network service provider or Nokia for repair options. Before sending your phone for repair, always back up your data, as all personal data in your phone may be deleted.