Hard Reset your Windows Phone

If your phone is unresponsive and you can’t reach Settings , you may be able to reset your phone using the hardware buttons. On Nokia Lumia phones (and some others), do the following to reset your phone:

1. Press and hold the Volume down and Power buttons at the same time until you feel a vibration (about 10-15 seconds).

2. When you feel the vibration, immediately press and hold the Volume down button until you see a large exclamation mark.

3. Once the exclamation mark appears, press the following four buttons in this order:Volume up , Volume down , Power , Volume down . Your phone should now reset and restart itself. (It might take a while for the reset to finish.)

http://www.windowsphone.com/en-au/how-to/wp8/settings-and-personalization/reset-my-phone