Why isn't my Alarm Clock Going Off?

I am using the alarm clock in the "clock" app by Apple, but recently the alarm will not go off. I wake up and go on to my iPod to see the pop up "Alarm" with the button "OK" to press and that is all. No sound, even with my iPod's volume all the way up. I did a test when I was awake and when it gets to the time I set it for, the box with "Alarm" and "OK" pops up but nothing else happens. What should I do? I am using iOS 6.Thanks!