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.
Try to minimise the apps and functions you are using at any one time.
In most cases it is normal for your phone to warm up, for example, when you are using multiple high-performing apps, listening to music for an extended time, or when your phone’s charging. The environmental temperature may also cause your phone to warm up.
Your phone meets Nokia’s high testing standards for device safety. Nokia also meets the international standard for safety (IEC 60950-1) and country-specific requirements.
To avoid getting too warm and to protect itself, your phone may automatically slow down, close apps, and, if necessary, switch itself off.
If there is no response when tapping the touch screen, check the following:
If the touch screen doesn't yet respond, try restoring the factory settings. If you cannot access the factory settings menu via the touch screen (Settings > Phone > Phone mgmt. > Factory settings), try entering the following reset commands via the keyboard. Before resetting the device, take a backup of your phone via Nokia Suite.
If the touch screen is not yet working properly, take the phone to Nokia care point for inspection.