Go to Start – Applications list – Settings. Then scroll to the bottom and choose About. Then press Reset your phone. Then press Yes.
An alternartive method if the phone isn’t responding is to try the following:
First press and hold the Camera + Volume Down + Power buttons simultaneously.
Then release the Power button when the phone vibrates – but continue to hold down Camera and Volume Down.
Then release the camera button and Volume Down when the Format prompt is displayed on screen.
Then press Windows button twice to confirm.