It could also be that the cdrom drives have burnt out... OR that you're trying to boot a DVD in a cdrom drive... Several options really to think about...
Best bet is though, utilizing a thumb drive (if the bios supports booting from a USB device) or swapping (temporarily) with a dvd drive that you know works (dvd drives are dirt cheap now adays, and since it'd likely be temporary, size doesn't really matter much).
Another item to check is to be sure the cable is plugged into the drive; I know I've disconnected devices that I've not used, so it's possible that the power/data cable isn't connected to the drive...
