Go to 'control panel' then 'system' then 'hardware.' Click on Device Manager and check your sound card, see if it has the ! symbol on it. If it does, you need to delete it. See if you can find a new driver for your sound card, reinstall the drivers and reboot. Hopefully after all that, it should work again. Plug your computer speakers into something else to make sure they are still working, maybe grab a portable CD player or MP3 player and plug the jack into that and see if it plays. If all else fails, you may have a faulty sound card.