You can try this data recovery software :
http://download.cnet.com/EaseUS-Data-Recovery-Wizard-Free-Edition/3000-2248_4-75184619.html
Or, http://zero-assumption-recovery.en.softonic.com/
If the external HDD can be seen by BIOS, then there is a hope of recovering data.
You can try to replace the USB cable, or can use other ports, see if t works. BTW does the external HDD spinning, or can you feel any vibrations by touching it ? If so, then the HDD might be ok.
The last thing will be to format and clean install of OS. As I said earlier the file system of the OS is got corrupted. Another thing you can try the external HDD on another system, see if that of any help.