you obviously need to re-install the sound driver

1. get a recovery CD that came with your laptop, put it in and see if you can find drivers on it

if not
2. go to manufacturer's/ vendors website (if your computer is dell, go to dell, if it's toshiba go to toshiba) find the support section, enter your computer model number, the website should provide you with drivers available for download. Download audio device drivers and install them.

If neither works, have your computer been subject to being dropped or other physical stress? how about power surges? did you run your speakers really loud on max volume?