"Headphones" output device disappeared?

I have sound on my computer, but when I try to plug headphones into the jack, the sound continues to come from the internal speakers as if I never plugged headphones in. I go to system preferences and look to switch sound to the "Headphones" option, and it's not there. I have tried numerous sets of headphones. I see no red light when I unplug them. I installed new RAM last Sunday, but I believe the headphones have worked since then. Does anybody have any ideas as to what could have happened or how I fix it? Thanks.