i had the same problem. I solved it by going to my soundcard settings and checking if the speakers were set to 5.1 sound. They were set to headphones so i put it back to 5.1. that might work, if not...
try hooking the speakers up to an external source such as an ipod or cd player, that will tell u straight away whether its a problem with the computer or the speakers. Have u had the volume at extreme volume recently? that could dameage the speaker cones, but u would still be able to hear a bit. Check to see the fuse inside the amp, or if it is a tube amp, one of the tubes may be blown, but this sounds more like a fuse.
Yea wat i did when one of my speakers wasnt working was to open it up and just check all the wires are fine. Sometimes you have to push em back in plus my amp had some probs too but good luck with your situation.
