A few days ago I was using my computer as usual and I turned it off. The next day it wouldn't boot up. It just stayed after the hardware diagnostic window on the intermittent underscore screen. So clearly It wasn't booting up. I tried turning it on and off a couple of times with no avail. Finally I used windows 7 disk and it seemed as if there was no HDD. not even the installation would see the HDD. So i thought it was dead, i bought a new one installed it with windows and used a external case with the old HDD. plug it in and still couldn't see it. I finally downloaded a partition program EASUS or something and my HDD was there listed WITHOUT a system letter. I could however explore it and i set it as Unhidden and it came back to life. I could see it normally.

I really just wish someone could explain to me what happened here, was it a virus? does it means the HDD is about to die? How can i prevent this or what should I do now? should i stop using this OLD HDD?

Thanks