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. Example for Nokia N8 and E7: Select Menu > Settings > Phone > Phone management > Factory settings > Restore.
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.