Ok thanks everyone for your suggestions... Now my hard disk is running fine. I don't know what had happened back then but here is what I think got it working again :
The problem was that whenever I plugged in my hard disk and tried to boot my pc, the pc wouldn't go beyond the
POST.
So what I did was I removed the power cord of that hard disk while I kept the data cable on and used Spinrite cd to boot. {
all of this while I had the other HDD connected}
then while Spinrite was beginning to boot, I plugged in the (damaged) hard disk and removed the old disk's power cord.
Once spinrite showed the screen to check disk
{or something similar...can't remember the exact terminology} and selected the option to check the disk in some basic mode or something {
that disk called it some option 2 i gues } which took about 10 minutes to complete.
Then Just to be sure, I restarted my PC and this time booted into Gparted {
again this time I'd removed my hard disk until I went past POST} and then again plugged in the hard disk once GParted screen was detected. This time I was able to see my disk listed there so I selected the option to clean up the disk and finally everything was alright.
Now with all of this, can anyone tell me what exactly had happened to the disk earlier ?