The first thing I would check is to make sure your computer is recognizing your sound card. Go into Device Manager and make sure it is listed there under sound controllers and also make sure there are no yellow exclamation points or question marks next to the device name. If so then there could be a hardware or driver conflict.
If your computer uses onboard sound which is built into the motherboard then make sure it is enabled in the BIOS. If you are using an actual PCI soundcard then the onboard audio should be disabled.
You can also check to make sure the Windows Audio service is started. Click on Start, Run, and type in services.msc. Scroll down to Windows Audio. Double click it and click on Properties. Make sure that Startup type is set to automatic and that the service is started.
Finally I would try to reinstall the sound driver. If your computer is a name brand such as a HP then you can look up the model online and download the appropriate driver.