Hard Reset Nokia Lumia 800

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, heres 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.

Reset Via Menu 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.