I troubleshooted a computer that had non-working sound a few years ago, and it was a pain. The first resort is to find the CD that came with your computer and find the option on it to reinstall your sound card's driver. If that doesn't work, you may want to contact the vendor who made your sound card. If that doesn't work, you may need a new sound card - they aren't that expensive. Caveat/confession: I never actually solved the problem on that computer a few years ago. But I'm a Mac guy at home, and we never have problems like that.
I'm assuming that you just aren't getting any sound, and you're not getting any error messages as well. Of course make sure everything is plugged in correctly. Open your volume control and make sure you don't have the sound muted. If it doesn't appear to be muted then right click on your volume control. Click on "open volume control" and make sure that there isn't anything muted there as well. Try right clicking on your volume control, then click on adjust audio properties. Click on the hardware tab, and it should tell you if your audio devices are working properly. If not then it should give you some type of message instead. Please try to let us know a little more detailed info about your problem, because there could be various different things causing it. Try to narrow it down for us.
does it show up in your system hardware? might have come loose under my computer, properties, hardware tab