Are you using XP?
Does the DVD/CD drive show as present in My Computer or has it 'disappeared' from the list?
If it is XP and it has 'disappeared' from the list in My Computer but is showing in Device Manager then I have a registy fix for it as I had this problem yesterday with mine and got it sorted.
I will post the fix on here when you confirm that the above is what you've got. It is only a little text file that you paste into notepad and save to desktop then merge it into registry by doubleclicking on it and then you reboot the pc and hey presto the drive is back.