You will lose all files after this process. Backup your files.
Lumia 520/635/710/810/925 etc
Turn of the device
Press Power button
When phone vibrates leave power button and press volume down button till you see exclamation sign !
Now press the following button combination
Volume up - Volume down - Power - Volume Down
Phone will reboot and you will see gear wheel spinning .
Wait for process to complete. (Can take about 8-30mins)
Phone will reboot after process is complete..
Lumia 800 / 900
Turn off the device
Press and hold volume down - Camera - Power button for about 10 secs
When phone vibrates release power button but still hold on to volume down and camera button
Phone will start reset process
Wait for it to complete.. Can take about 8 - 30mins