Why does my sleep alarm can't turn off?

Hello,

If your iPhone alarm isn't going off, the most likely reasons are: your phone volume is too low, it's on silent mode, you have Do Not Disturb mode enabled, or your "Attention Aware" features might be lowering the alarm sound; double-check your alarm settings to ensure the correct time and days are selected, and that you've chosen a sound for the alarm. 

Good luck.