Pretty rare for a soundcard to just up and die, and more often than not they're just a chunk of the motherboard anymore.
Hard to say. I'd check the obvious stuff first, mute, speakers on, plugged in, yada yada.
On a somewhat related topic, I once had a guy at work call me up and say the sound on his computer didn't work, come right away.
What he meant was that the sound OF the computer was "wrong", and it was hosed. HD was going GRRRRRR.