Try using the drive to boot the machine. You'll need a bootable disc, like a Windows installation disc, a live Linux distro, or even something like the Ultimate Boot CD. Also check the bios of the computer and make certain that the first boot device is set to the optical drive and not the hard drive.
If the computer boots into the disc, rather than into the operating system on your hard drive, then you know that the drive is functioning correctly, and you have a software issue. If it won't boot into the optical drive, and you are certain that the boot order is set to boot from the optical drive, then your drive is probably dead, and needs to be replaced.