If the sound in your computer has stopped functioning, many possible issues depend heavily on the sound hardware your computer has. However, you can take some basic steps to restore your computer's sound.
Check to see if the mute button either on the keyboard or in Windows has been accidentally selected. You can type "volume" into the search field of the Start menu and click on "Adjust System Volume" to check if the system is muted or the volume is turned down.
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Make sure your speaker cords have not become loose or unplugged and that the cable is securely attached to the proper jack. Try different cords if possible to ensure the cords aren't bad.
Try a different set of speakers to see if your speakers are bad. You can also try headphones.
Sometimes the computer does not have sound because the drivers to the sound card become uninstalled or corrupted. You can check to see if there is anything missing by typing "device" without quotes into the search field on the Windows Start menu and then clicking "Device Manager" in the resulting list. Click the little arrow next to "Sound, Video, and Game Controllers" to drop down the list underneath. Ensure there are no yellow icons with exclamation marks in them next to any of the sound-related items. If there are, it means the sound card needs to be reinstalled or is not recognized. Use the disc that came with the sound card or download drivers from the Internet to restore functioning.
If your sound is still not restored, consult a computer professional.