You can try a system restore, to an earlier date when u know the sound was working.
If not, you have to check whether the driver for sound card is installed or not. You may try to update the driver and see if it works. For that you will have to know the make and model number of the soundcard. Hope you know how to do that.
A similar problem was there with another person, and its been discussed on another thread. Will give u the link. Many people had this problem, and there is not a specific solution for this problem.. so you will have try out the different solutions and see which works for you. Go through the thread with patience, and try to find which solution works for you. Heres the link of that thread:
http://forums.techguy.org/hardware/7...i-have-no.html
You may have to be patient on this one.
But since you told about sound card, i am guessing it must be something to do with the driver.
If none of the solutions work, reinstall may be the only option left.