Computer breaks, but which part?

My desktop computer has been giving me trouble for the past four hours, and besides wasting time I wanted to use for learning, I’ve mostly had my patience tried by my inability to determine which part is actually broken.

• at first I thought it was the graphics card (black screen), so I get another one from a neighbor
• then it doesn’t start (or starts and stops again a second later), so maybe it’s underpowered
• different power supply in, still doesn’t work
• I discover the screen cable is loose, so that might explain the black screen
• unable to fix the problem, I decide to connect my notebook to screen and keyboard and use that
• however, mounting the old hard drive as an external USB drive does not work because the drive refuses to spin up no matter what I do

So, I’m using my notebook right now with my actual external HD. The drawback is that I’ve lost current tabs and some video and audio data that was only on the desktop’s hard drive. I have to see if I can get it working again. Now, I should use the rest of the night to learn something.