I've seen this happen all too many times recently with lower quality USB2 enclosures. Depending on the chipset the enclosure is using these errors may occur. What happened most likely, is your file table got corrupted. The data should be intact, but you will need to use a third-party recovery tool to get it back. I think Maxtor's tools provide some functionality for recovering the data (they sublicense Ontrack DataRecovery software). There is a good free utility available
here. If those don't help, I have had very good luck with
GetDataBack series which is not free, but well worth it.
Please post your progress.