Well, I’d plug it to a standard PC and see wether its even mountable. What filesystem did you use? Ext3? Then chances are alright, given the journal and stuff, I’d say.
If it doesn’t even mount, you have to remove it fro mthe USB enclosure and put it to a standard (P-ATA or S-ATA) connector in a PC. Other than that same game.
So maybe the USB enclosure has died, but the HD has not. I’ve seen that.
In Unix you can just dive in with fdisk and the likes, to apply a few checks to see wether data is there, anyhow.
Can’t tell you what tools to use henceforth, though.