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:
Press and hold the Volume down and Power buttons at the same time until you feel a vibration (about 10-15 seconds).
When you feel the vibration, immediately press and hold the Volume down button until you see a large exclamation mark.
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.)