THE USB THUMB DRIVE IS THE CULPRIT!
So one day our Lenova All in One PC greeted me with nothing more than the words “operating system not found”. I was all, “Well, have a look around! It couldn’t have gone far!” The DVD drive was empty.
Ultimately I hit F2 on bootup, and reloaded my operating system from the backup partition. It still didn’t work! I gave up and went to bed. Just as I was in that problem-solving state of pre-sleep, I remembered that we keep our Point of Sale software on a USB stick. On awakening, I booted it without the USB stick, and of course it works fine now.
Apparently this computer is too dumb to continue looking; the first device it encounters, it looks for the OS, and upon not finding it, tosses up an error message and goes back to sleep. It makes me wonder if computers assume the habits and personalities of their owners.
P.S. Later, this gave me the idea to hook up a USB hard drive I have laying around, and install Linux on that; I can boot to either without any messy drive re-partitioning or whatever. I always wanted to try Linux. Then again, I own a CB radio.