Windows 8 - No Boot

So this is going to sound a little strange, but I will put it as clearly as I can.

I'm working on a gal's computer (HP 2000 laptop). Her problem? When she hit the power button, the computer would not boot. No BIOS screen, no boot screen, no HP screen. Nothing. Just a blank, black, backlit screen with no activity on the HDD indicator LED. If I remove the hard drive, however, then the computer will boot to the point of giving me a message instructing me to install a hard drive.

So you'd imagine the issue is with the hard drive.

I hook her HDD into a free slot on my desktop to check the device. I'm able to access all files and it checks out okay. But if I put it into her computer, all you get upon touching the power button is a blank, black, backlit screen.

I'm assuming at this point that the boot partition on her HDD may be corrupted and a reformat and clean install are needed. I was able to back up her personal folders. However, there is no COA sticker on her computer (thus no Windows Key), just a "Windows 8" sticker. She said her computer did not come with a set of discs either (HP puts recovery data on a second HDD partition, which I am unable to access at this time). I asked her to contact HP and request a set of recovery discs for her computer, so that is where the process is at right now: waiting for a reply from HP.

I'm curious: based on what I described above, would you also assume the boot partition on her HDD may be corrupted, or would you come to a different conclusion?

Might be a Motherboard Issue on the Laptop - You got an extra Laptop drive? If so, plug one in, and see if it detects the drive. If it does the same thing, Then the System is not detecting the drive. If it tries to boot, then it's the Original Drive.

Tried changing the motherboard, but having the same issue. For some reason she thought that the motherboard was the issue so I went ahead and ordered one but the same thing is happening. That's how I've arrived at the probability of it being a hard drive issue.