Do you know what happened? Does the drive hardware work? Does it mount but just not show files? I'm curious if the partition table is lost or if the drive even spins up.

I've had it happen where an external drive was unmounted incorrectly in OS X and all the data disappeared, even though the drive showed up. Unmounting, rebooting, and remounting the drive a few times made all the files magically reappear, oddly enough. I'm not sure, but I suspect the journal might have gotten corrupted and maybe the drive was fscked (or whatever macs do to integrity check) on reboot.

Try using the command line on your mac to see what is in /Volumes/yourdrive. Perhaps something goofy is keeping the finder from correctly loading things, but the files are still there. Another thing you might try is connecting it to a linux box and running fdisk to examine the partition table. See if you can mount it there and if anything exists on it.