With the computer shut off (disconnect) the power and data cables from the problematic DVD drive then reconnect them (making sure the connection is snug). Do the same to the other end of the data cable where it plugs into the IDE connector.
Reconnect power and reboot. If drive is seen no problem. If not seen try the above with a new 80 wire IDE cable (they can turn bad).
If still no joy, time to check into the warranty on the drive (three month old system should still be under warranty).
I have XP Pro, Athlon 2500+, 1024 MB RAM. I've had my CD-RW and DVD-ROM (everything on my computer is less than three months old) work fine until yesterday. Then the associated drive letters no longer showed up in explorer. I restarted, to have it freeze on the "Detecting IDE drives...". I restarted again and went into the BIOS, noting that now the CD-RW shows up. Now the CD-RW (on the secondary Master) is showing up and is fully functional, but my DVD does not show up, nor does it respond when I try to open the drive.