NTFS is a very reliable filesystem, so it's far more likely that something went wrong with the hardware. (If the drive is formatted with FAT32 it could be just about anything.)

But the point is that if you really haven't done anything unusual (like repeatedly trying to hot-swap the drive while the system is running), then the drive is almost certainly dieing, and you've got a small window to save your files.

Anything you do to try and "fix" the drive is likely to just mess things up more. Don't take any chances, and recover all the data you can before trying anything.

Once you've got the drive backed up, "chkdsk" is about all you can do, and you're likely to end up with a whole buch of lost files segments, which are a real pain to sort out. "chkdsk /f /v /r d:" or whatever your drive letter is.

I'll bet that if you look in the event log you'll see a lot of harware level error messages.

--Nathan