Hi,
First try to restore ur system on the point just before u lost ur sound :
Start/All the programs/Accessories/System Tools/System Restore.
If that doesn't match, u'll have to re-install ur sound card.
Tell the brand and the model of ur computer (or of ur MB if ur computer is a no-brand one) and I probably will be able to give u a linK for free downloading the driver u need.
Errare humanum est, perseverare diabolicum.