Welcome to the community, Arun_09!
I'm sorry to hear about your issues with the WD external drive!
🙁 If it appears as unallocated in Disk Management, it means that it doesn't have a file system. Do you have any important data on that drive that was not backed up elsewhere?
I'd recommend you use
WD's Data LifeGuard Diagnostics for Windows and run the QUICK and EXTENDED tests from the utility in order to be able to determine the health and SMART status of the external HDD.
IF you haven't tried that already, I'd also suggest you use a different USB cable with the hard drive and also attempt plugging it to a different PC/laptop just to check how it will be recognized there.
Keep me posted.
SuperSoph_WD