I was out of town all week, so apparently just sitting in the garage was enough to run down the puny 12-volt battery inside my car. A Prius actually has two batteries: one to run the car (of course) and the other to run the electronics -- and that's the one that was dead. Next time I guess I better manually turn off the keyless entry system. Whatever the cause, it wouldn't start this morning. One problem with a Prius is that the all-electronic drive means you can't even get it into neutral without the 12-volt battery, which in my case meant I couldn't get jumper cables close enough to start it myself, so I called AAA.
The service guy who arrived was friendly but pretty clueless about the Prius. He eventually agreed to look at my manual and long story short, we got it running.
Now I've had more time to look into the situation and I wonder why next time I don't just attach any old 12v battery. Even the Toyota web site says:
Q: Can Prius be jump-started?
Yes. Should you need it, Prius can be jump-started with any standard 12-volt DC power source, and it actually requires less power than a conventional car. Simply connect the cable clamps to access points under the hood (which are connected to the auxiliary battery near the cargo area) and energize the computer. Then press the Start button to turn the car on.
In other words, a simple 12V power source, like the one in one of our emergency lanterns (or maybe my laptop?) would have worked fine. Next time I'm going to try that before wasting time with AAA.