Hey there, MaishaM.
Sorry to hear about the issues you're experiencing with your HDD. :\
Have you had any other problems with it before this happened? Any unusual sounds/noises coming from the drive, unusual transfer speed, freezing/hanging the system when you connect it? Did you use any specific program to backup the files or was it a simple copy - paste?
Would you be so kind as to post a screenshot of what you see in Disk Management when you have the drive connected to the computer?
I'd recommend that you try the HDD with a different USB cable (preferably not longer than 12"-18") as well, to see if the issue still persists.
Please let me know how it goes.