Please tell us more:
What make/model computer, what sound card, what sound outputs, what software subsystem (alsa, pulse, and so on), etc.
What do you see when you check the mixer settings?
Is there a hardware control that might mute the sound (hardware controls can be sneaky--I lost my wireless for a month on one box because I didn't realize there was a hardware key to turn it off and on)?
For a test, I would recommend booting to a Live CD of something and testing the sound. If it works under the Live CD, it's likely software. If not, it may be hardware. Also, be sure to test with headphones. If this is one of those desktops with a sound card with multiple output ports (surround sound, woofer, etc.), test all the ports.