Sorry about your computer problem and the other forum problem you had!
I've built (from scratch) and repaired many computers in the last 20 years, and in my experience, the first component to die is the power supply. The second component to die is the motherboard, in which the capacitors will blow out and be rounded at the top, instead of flat. The third component to die is the hard drive.
If the computer won't turn on, it could be the power supply or motherboard. I always replace the power supply first to see if that fixes the problem.
If the computer boots up at the beginning, and if there's a hard drive problem, you'll see error messages at startup relating to "hard drive not found" or a similar message.
Since your keyboard quit working at the beginning, I'd hook up a different keyboard first. If that doesn't fix the problem, I'd replace the power supply with a working one. If that doesn't fix the problem, I'd suspect a defective motherboard. Sometimes, motherboards will die without any "blown, rounded-top" capacitors, so it can be difficult detecting what part of the MB is not working.
Undoubtedly, though, there is probably only one part that quit working on your computer. The hard drive is probably still okay. It's just a matter of elimination to find the broken part. I have a lot of old computer parts (saved for years that I know I'll never need....dumb), but I'm not very close to L.A. or I'd bring you some items to test.
Good luck!