I'm not sure how it happened. I assume that coolant just disappears over time. You know, I did assume there'd be a warning, given off by the rogue Windows ME machine that runs my car. I suppose the "warning" was when the temperature gauge would rise, and the car would give off some dire and cryptic warning about engine temperature.
Maybe it was hyperbole to say that there was no coolant. I could see that the coolant reservoir was bone dry. There may have been some in whatever other part of the engine that stores coolant - the radiator, I guess?
But, I added coolant, and the temperature issue went away for now.