The other night I decided I would test out plugging my guitar amp into my pc and seeing if it would detect it. It did, and I used it to record on Audiocity without having to use a mic. But then today, my PC no longer has sound. I've checked all of the volume control options and nothing is muted. I can still use my PC speakers as an amp, but I don't get playback or any other usuall sound that I would get. And when I press mute on volume control, I hear this loud buzzing.

Can anyone tell me what the problem is?

So you plugged your amp into the Line In of your PC to record, right? It wouldn't have 'detected' your amp, anything you plug into your Line In is goin to work (if it has a signal).... anyhow, it's quite possible you popped your soundcard dude. Amp's have a lovely habit of spiking, popping, and generally making a lot of evil noise.

Your soundcard won't be capable of handling pops/spikes from a guitar amp, cause they operate a lot, lot louder then your average soundcard. So if the amp has been plugged into your PC and you've pulled out a lead, or stepped on a cable or something like that, POOF! Goodbye soundcard.