What does the Apple System Profiler say?
If it doesn't show up there, either, then your drive is, quite simply, broken.
DataRescue X might conceivably be the last software option to glean data from the drive - it's very unlikely to work, though.
the only option then would be to remove it from the Firewire enclosure and see if it works in another, or if you stick it directly in a machine. Note that this may void your warranty.
If that doesn't work, then it's off to professional data recovery firms, if the data is very valuable to you (but only then $$$)