Screwing the computer parts back together is the most dangerous step
I had removed the cover from one of the computers in my office in order to upgrade one of its hard drives (from 20GB to 200GB, woo-hoo). The hard drives are kept in a removable cage, so first I had to unscrew the cage, then unscrew the drive from the cage, then swap in the new drive. Of course, you don’t screw everything back in until you’ve tested it out, so I had the computer running with its innards strewn about my floor until it ran to my satisfaction. Okay, time to put everything back together. I screwed the drives into the cage, screwed the cage into the case, but before I screwed the cover back onto the case, I turned on the computer just to make sure everything was still okay. A colleague of mine happened to stop by as I was doing all this to discuss a technical matter, and we chatted about the problem while I sat on the floor with a screwdriver. I said to my colleague right before I turned the computer on, “Look, I bet it won’t work.” Lo and behold, the computer didn’t work. It just made a horrible beeping sound. Screwing the computer parts back together is the most dangerous step in computer assembly because once you do that, there’s a pretty good chance that something will stop working. I spent the next fifteen minutes re-disassembling the computer, removing and re-attaching every cable that might have wiggled loose, all to no avail. Eventually, I found the loose connection: While mashing the cables around, one of them accidentally pushed against one of the RAM release levers on the motherboard. As a result, one of the RAM sticks was not fully-seated.
Push the RAM back into its socket, power up the machine, everything works again.