The sound is not working for usually three reasons.
First someone or a program turned the volume all the way down or if external speakers they were turned off or unplugged. To check your volume setting in Windows 7 go to the lower right corner and click on the icon that looks like a speaker (side view)and then adjust the slider to increase volume. You can do this by going to Hardware and Sound in Control Panel and then adjust sound settings.
The second reason is that the volume has been muted. This is undone by going to same place to adjust volume and unchecking the mute box. Muting can be a good feature such as when you browse the internet and do not want all the videos on sites like CNN that auto play to start coming out your speakers.
The third is that you installed a bad driver or an update caused sound to disable. The simplest way to fix this usually is to do System Restore which you can access thru Control Panel and set the system before the update or change occurred. You can also go in the Control panel to System and Security and to Device manager and then to sound and video controllers and choose your sound controller and check drivers and try to update or rollback. Usually a system restore will cure problem if one and two did not.
It is amazing how often when I get this call that people discover the speakers have either been muted or turned off or unplugged.