Regarding the hard drive cable which Cory mentioned, note the location of the connector in the photo with Step 7 in that link above. Maybe you could "trace" that to its source, and try and replace it. Not sure how difficult it is, but I remember that for my Mac Mini, I was having a problem with one of the connectors, and the end that attached to the motherboard was an issue. Replaced it (easy job, and the cable was inexpensive), and I was back in business.
Also, you should follow that guide to remove the drive, place it inside an external case (those are not expensive), connect it to another Mac, and see if it shows up. You could then try and use Disk Utility to repair the drive (from a software perspective). If successful, you could even recover data from it. And, if desired, use that drive (as long as it is "healthy") for back ups, storing stuff, etc.