Hi sartranew,
A couple of things you can try one at a time are, preferably in this order,
Go to device manager and check to see if there are any yellow warning signs showing, if so fix them by installing correct drivers etc.
Check you can see your sound card in device manager under "Sound, Video, and game controlers".
1) If all is well in Device manager, surmizing it is a desktop, remove the side and then remove the sound card.
Blow out the PCI slot it comes out of, and also blow the card itself down (can of compresed air would be ideal if you can get hold of one). (Ignore this option if it is a laptop)
2) Uninstall the sound card in Device Manager, reboot the PC and let it install itself again.
3) Find the sound card make and model, download the latest driver from manufacturers website, uninstall old one, and reinstall the new one.