We'll need more details. What exactly is it doing? Has it recently happened, or did it never work?
Does a light come on at bootup? Does the door open/close? What happens when you do put a disc in?
Hope I can help.
NOTE:
If you've had this problem since you put it in, try turning over the IDE lead, as some leads don't have a marker suggesting the right way up. If its in the wrong way, it simply won't boot, and it'll cause problems.