Since your disk is showing up with a drive letter, hopefully, check disk (chkdsk) can help:
http://www.wikihow.com/Fix-a-Cyclic-Redundancy-Check-Error
(this if for Windows XP) Google "checkdisk windows [your version]" if it doesn't translate well.
Falls are a huge cause in external hdd failure, though, so I wouldn't be surprised at all if the disk is beyond help. Try connecting to another computer (to see if the problem still occurrs) or another operating system (you can boot to a linux live cd) or, at least, use another USB drive on the computer you are connecting to.
Next step would be to *cross fingers* hope the external enclosure suffered damage, but that the actual drive did not. If you are comfortable doing it (likely voiding your warranty), disassemble your external enclosure and remove your SATA hdd. You can open your PC and connect it to your motherboard assuming you have an available SATA connection, SATA cable, and available SATA power hookup. Alternately, you can buy a 'kit' to connect a hdd to your pc via usb, which prevents you from having to open your PC. It is much more convenient if you can diagnose without purchasing 1-time use hardware, though.
(Google "sata to usb" -- i buy from NewEgg usually, cheapest with 4 stars)
You can try data recovery software, but they haven't been very helpful to me in the case of physical trauma to drive.
Data recovery outfits (expensive) may help if the information is not otherwise recoverable.
Good luck!