Computers can be changeable and unstable so if your computer suddenly stops producing sound, you need to know the reasons for it and address the problem properly. If you have money and time to spend, you could go to a professional computer technician to resolve the problem but you can also do some troubleshooting steps yourself.
A computer stops producing sound sometimes due to the uninstallation of the drivers to the sound card. What you need to do is to restore them. First, examine if the drivers are missing by opening your computer's Control Panel. You can press Ctrl + Esc to see the start menu and find the Control Panel at the right side. Then, click on System and click Hardware. Under this option, click Device Manager. If you find yellow exclamation marks beside the sound icon, that means that the sound card has to be restored since the drivers have been corrupted. You can take out the CD which came with the computer sound card. Alternatively, you can restore the sound card by downloading drivers from the net.
However, if you see no yellow exclamation marks, you don't have to do the step above. Check if the computer speaker cords are not loose nor unplugged and make sure that the cable is properly attached to the right outlet. You can exchange a few various cords. If this works, your computer sound will be restored.
If the steps above still don't work, you should examine the mute button on the Windows Taskbar and double-click the Windows Volume Control which looks like a speaker. Check if the sound has been muted. If it has, uncheck the box to unmute it. However, if you can't see any sound icon in the Taskbar, your sound card may need restoration. Use the CD which came with the sound card or download the appropriate drivers from the website of your sound card's manufacturer. This would solve the problem.
Also, you can change your speakers if you find that they are defective. Don't forget to keep the volume control up.
Source: ezinearticles.com