This could be a huge range of issues (I’m assuming you have Windows because you mentioned your volume icon is on the right side of the screen):
1) Ensure your speakers/headphones are plugged in. My old desktop computer didn’t have a built-in speaker; if I would forget to plug in my headphones, it would tell me I had no audio device.
2) Restart your computer. With Windows, sometimes a simple restart can work wonders for mysterious problems.
3) Reinstall/update the driver for your sound card. Go to the Device Manager, find your sound card, and Google for the driver. BE CAREFUL: only download drivers from the manufacturer’s web site.
4) Did you make any changes to your computer recently, such as installing programs or cleaning the registry? Try using Windows’ System Restore to bring your computer back to its previous state.
5) If all else fails, it might be a hardware issue.