Lumia 720 stuck on lock screen alarm

[K71st]

This occured when the phone is alarming . I didnt had the chance to turnoff the alarm because it was stuck on alarm.I tired pressing down lock and volume together but nothing happened. I didn't install any software regarding lock screens. Its windows 8.1. please help!

 

[berty6294]

Hold the volume down and power button in for about 15 seconds or until you feel the phone vibrate.