Try a SOFT reset first. It won;t erase any of your setting or date : Hold down the POWER button + VOL DOWN at the same time until the phone vibrates (15 sec).
If that doesnt work you will have to do a HARD reset which will erase your setting, apps and data (If you backed up your phone you can RESTORE it)
(From the MS site )
[h=3]To reset an unresponsive phone[/h]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.)