With most device you would be able to pull out the battery and a soft reset is done, but with the Nokia Lumia 800 you will have to do something differently, here's how:
1. Press and hold the
Volume Down and
Power keys until you have it vibrates 3 times, then release all the keys.
Hard Reset
When soft reset does not do the trick, then you ought to use this option. Please note, if you try this option make sure that you have backup all your data.
1. Proceed to backup your valuable data.
2. Next turn off your phone.
3. Now press and hold the
Volume Down, Power and
Camera key simultaneously until you feel your phone vibrates, then release the
Power key and continue to hold the
Volume Down and
Camera key for a further 5 seconds.
Now your phone should start to return to the factory default settings.
1. Go back to the
Home screen.
2. Click on the right arrow
button.
3. Then scroll down the screen and select
Settings.
4. Now scroll down and select
About.
5. Then select
Reset your Phone near the bottom of the screen.
6. Tap on the
Yes buttonwhen the warning screen appears.
Now wait for the device to reset it self.