MacBook Pro speakers not working.

I was using my (mid-2010) MacBook Pro running Lion and then started watching a video and suddenly the internal speakers stopped working. There was no red light coming from the headphones jack. The volume was not muted. I tried plugging headphones in and even that wasn't working. I can adjust the volume but the speakers or headphones don't work. The headphones are working, I'm certain of that.

What should I do?