might be a bad sound card, but make sure mic's plugged into line in (not headphones) port; it's green, but if sound card is really old, you might have to try other port. Then go to control panel, Sound and Audio, and make sure "Mic in" isn't muted (it often is, no idea why, maybe security).
otherwise, might try another mic to make sure it's not the mic that's bad (they can be junk).
finally, if a few mics fail, I'd guess sound card is the problem. (They've made really cheap ones in the past.) If you're attached to your computer, maybe look in a pawn shop or 2nd-hand store for a sound card. You'd be surprised at what people pawn this time of year.