Hey there, balaji006!
Most probably your external drive had a hardware encryption and that's why you cannot get it to respond in any way.
Or if you've opened the HDD itself, you might have made some component become polluted which would be a physical damage to the HDD. The only way to get your data back is to contact a data recovery company and explain them the problem with your drive and see if they will be able to do the recovery. You should know they are not cheap though.
I would just recommend you to get a new external and always do a backup of your data.
Hope this helped. Good luck with it!
SuperSoph_WD