My guess would be that you pulled the drive prior to a write being completed. Select the safely disconnect drive from PC and select the ext. drive. Then, power down the external device by disconnecting from its power and from the PC. Power down PC. Replug power back into both power and PC and turn on. Boot up PC. See if it fixed itself.
First I like TestDisk
and LatiosXT is the one who brought it to my and others attention on another thread. There is a good bet it will work.
Before that though I would do a sector by sector backup. A sector by sector backup requires a empty formatted drive or partition at least as big as the drive being backed up and will take hours to backup.
I would also try another USB (assume) cable if you have one, and port hub.
Alternatively you could first see what Recuva
sees. If it can't see anything, I'd try Testdisk.
If it fails, you can try a paid recovery service (free to check if it can see your stuff) like NTFS Recovery
or GetDataBack for NTFS