Step 1: Turn it off, wait a minute, and turn it on again. (It's amazing how often that works!)
Step 2: Try playing a CD. If there's no sound you probably need to reinstall the driver. If you do get sound then it's likely to be a problem with Adobe Flash Player.
Step 3: Reinstall the driver, or Flash, as appropriate. If you need help with the sound driver, please tell us the make and model of your computer. If it's Adobe Flash that you need to reinstall . . .
(i) go to your list of installed programs and remove all references to 'Adobe Flash' ;
(ii) restart your computer. (That's important!) ;
(iii) go here:
https://get.adobe.com/flashplayer/
(iii) deselect the 'optional offers', click 'Install Now' and follow any instructions you see.