2 year old compaq computer all original parts -- HD not found anymore

Hoping I don't need a new hard drive.

Received Compaq computer CQ61, With Win 7, 64 bit. If I try to run the Hard Drive test, I get "Hard Drive Not exist". (The computer's words, not mine) Really don't want to buy new hard drive, of course. This drive is only 2, but original equipment and no longer under warranty.

I'm told to run certain programs to check the drive. However, they have to be downloaded to the computer in question, and I can't connect via that computer. I created a bootable Flash drive, but it doesn't let me get to the hard disk.

The Disk is a SATA (by Seagate) 320 GB, Momentus 7200.4.

I can't partition when it can't see the drive, I can't run programs when I need the Hard drive to store them. Looking for some way to save the drive for a while while I save enough for a new drive!

Anyone with suggestions?

To answer your questions:
1) Yes, I opened up and checked. I don't see a power connector( wired) but I'm guessing it goes through the drive connection. If not, that's my problem.

2) Anything that is plugged in shows. DVD-CD ROM shows in BIOS. Flash drives show if plugged in. But NOT the H/D.

3) I can only assume the drive is spinning: I cannot hear. But the drive runs warm...warmer than what I'd expect.

Tried a hard drive tester...unfortunately, I get a "Cannot create MSXML INSTANCE". It DID work on another computer. And I cannot get a LAN to work between these computers I have.