We need the motherboard manufacturer, model # or if the computer is an out of the box type (Dell, Gateway, E-Machines) we need that number too.
also, check this basic idea:

Is the master volume or wave muted?
In your Control Panel, do you have any sound drivers?

another way to check to see if you have any drivers:

Also, a small program that will help you tremendously is PC Wizard. It will tell you more about your computer than you really wanted to know.
http://www.cpuid.com/download/pcwizard/pcw2009_v190.exeDownload and install.
Run after the install. Click the 'Hardware' tab on the left. On the right you will see the mainboard manufacturer.

You give us this info, we can get you the sound drivers. Unless the onboard sound has fizzled out on you. That is unlikely to have happened, but it is a remote chance.
Now, do your part, give us the correct info, we'll be glad to help!

Fordman