The optical drive may be defective or have a loose connection to the motherboard causing it not to read the disc.
If the optical drive is bad then you will need to replace the drive. Check the troubleshooting page here: [[Troubleshooting HP Media Center 876x Desktop PC#Section_Bad_power_supply|Troubleshooting Page]]
If the connection is loose then open up the computer case and reseat the cable that connects from the motherboard to the optical drive. See troubleshooting page: [[Troubleshooting HP Media Center 876x Desktop PC#Section_Bad_power_supply|Troubleshooting Page]]