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.
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 arrowbutton.
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.
nokia 500 hard resetNokia-c6-01 Hard reset ....
(vol+) + lock key + green key + power on button(red key)
problem: unit nokia hang
You have to press 4 different buttons for this to work:
1. Turn Off the device
2.Press the Volume Down button
3.Press the Camera Button
4. Press the Menu Button
5. Last step is to press the Red Power/End Call button to initiate the wipe
6.Keep holding the four buttons until you see the Nokia Logo
When you see the welcome message it means the Hard Reset was successful.