The first thing to do is check whether they are recognised on Boot-up. Assuming that you are looking for them, they should be found during the first screen and identified for what they are. If not, assume that either the power or data cables may have come loose and check - also make sure they're the correct way round. If all is OK, you may have a cable failure, so try replacing the cable. Also try with other power connectors - if one has failed on the same data cable, it could bring down the other. Also try each one as master.
If they are recognised, I would suspect a driver problem. Try booting with a floppy disc with the drivers installed (98SE or later) and test in DOS by typing the dir X:\ where X is the drive letter of each CD. If still no joy, report back giving details of OS and I try to take you further.