Lumia 920 Soft Reset:
Press and hold the power- and volume-down buttons pressed simultaneously for ten seconds. The phone should then vibrate and reboot.
Lumia 920 Hard Reset:
If a soft reset does not solve your problem, a hard reset (back to factory settings) might be your only option. To perform a hard reset press and hold the power-, volume-down- and camera buttons until the phone vibrates. Then release the power-button and keep the other two buttons pressed for another five seconds. The phone should reset to factory settings.