Check your volume controls - it may sounds obvious, but worth a check. I've used a program before which altered the Line In volume. When I quit, I got no sound from my TV card until (hours later) I opened that program and put that slider back up. Then I realised I could have tweaked the Line In slider in the Volume Control panel...
Have you tried different speakers/headphones? Or plug an MP3 player/radio/other computer into your current speakers? This would eliminate whether or not your speakers are working. I've had two sets of speakers just go, with no warning, and for no apparent reason.
Otherwise, as GVBN said, uninstall and reinstall your sound card driver. Your driver shouldn't just stop functioning, so I don't think this is a problem. But computers can act funny sometimes...
