These simple steps can help in a majority of cases.
Restart your phone
For phones with removable battery:
1. Switch your phone off
2. Remove the battery.
3. After a few seconds, put the battery in and switch your phone on.
For phones with non-removable battery:
1. Press and hold the power button until the phone vibrates and switches off.
2. Switch your phone on again.
Reset the settings
Restore the original factory settings as instructed in your phone's user guide.
Update the software
Make sure your phone is running the latest software. For downloads and assistance, go to your local Nokia website's support page.
To restart a device having integrated battery (e.g. Nokia N8-00, E7-00, N9, Lumia 800), press and hold the power key for about 8-10 seconds until the device switches off. Then you can switch your device on again normally.