It is possible that your sound card driver have been uninstalled. In order to fix this problem, you can go to Start -> Control Panel -> Device Manager (for Windows Xp) or Control Panel -> System -> Device Manager (for Windows 7), right click on the Sound controller and select Properties, go to 'Driver' and hit update. If after this, you still have problems with your sound, you can Uninstall the driver from the same page, then you can visit the manufacturer official download and support website where you can find and download the appropriate driver for your computer.
Also, you can try using a third party tool that will automatically download and install the appropriate drivers for your computer. I suggest you check Software Informers web page in order to find a program that can help you.