Paddy covered most of the possibilities.
Is this the only disk you have? Can you borrow or buy another to see if that one is recognized, if so, the disk itself may be bad, though I would think you would get one of those "you have inserted a disk this computer can't read, yadda, yadda".
You don't by any chance hear any clicking sounds do you? The older SCSI zip drives were prone to the Click of Death but I think the USB ones have been pretty free of that problem.
Here's an info doc at Apple but it's mainly for SCSI zips. Might help.
zip drivesI would suggest zapping the PRAM if it was an older Mac, but....
One slight possibility is the option to show all disks on desktop may need to be reset. I'm not on X so I can't tell you exactly where in the prefs that option is.
Someone?
OH, forgot
TO TS.