Well, sometimes the most basic points need to be reviewed. Do your speakers have their own power switch? If so, are they turned on? Do your speakers have their own volume control? IF so, have you tried adjusting it?
Are you absolutely sure your speakers are connected to the right connector on your computer? If your computer originally had onboard sound and you later added a sound card, you'll have two sets of connectors for speakers, mic, line in, etc. The onboard set would not work if onboard sound is disabled in the BIOS.