No sound after headphones use

I'm on OS X 10.7.4 Used headphones, unplugged headphones, no sound out of speakers! Yes, red light is shining through headphone port. Yes, I've looked at other discussions and followed instructions in system preferences and utilities--still no sound. Yes, I've restarted my computer--again, no sound. Can someone please advise how to fix this? (It's pretty frustrating to lose sound just because I've used headphones. Why does that happen?)