geeze that could be a thousand different things. Let's start with the first time:

Is it recognized in the my computer section of your os? If not it could be the Daisy Chain, the grey wire thing that goes from the drive to the motherboard or controller (or) sound card. Next it could be the drive itself being bad. It could also be a problem with your power supply or power cord, not the black or grey cable that plugs into the wall, but the one that is attached internally into the dirve. It could also be the drive jumper settings if the drive was a new install.

Please be a little more specific so we can help pinpoint what is going on here. Did you just install the drive? Upgrade from a new drive? Just turn on your old system and it stopped working? Upgrade, update or reinstall any programs lately?

Looking forward for you answers!!!

Also what kind of DVD Drive is it and what specs are we looking at as far as OS and system?