I work at a Student IT Help Desk at my school. and from my experience if your computer is unable to interact with the HDD, or it takes a long time to respond, it's usually a lost cause.
One solution that has RARELY been seen to work is to completely pull the HDD from the external harddrive casing.
Then plug the HDD into a computer via SATA cable, or into an external docking station if you have one available.
Then if you can't see it in Windows you can try Ubuntu.
If you still can't see anything, it's not recognizing, or it's beeing very unresponsive, than it's most likely a dead/dying harddrive.
At that point there are a couple of different companies you can send the HDD off to for data recovery, but it wont be cheap.
Anyways best of luck to you and I hope you find a solution for your problem!