Thank you, Robert. I went back to the car and kept pushing the Power button. It finally turned off. But now I can't recall if I had my foot on or off the brake. But I will keep your suggestion in mind if this happens again.
I noticed something else when I went to try again: the "12-volt battery charging system indicator" was illuminated red. Now, I've been playing "car roulette" with that battery because it's still the original one from December 2017 when I bought the car. (And when I looked at the indicator on the battery itself, there was no blue circle, only white, indicating "needs charging.") If everything starts up in the morning, I'll head to my dealer to see about a new battery.
Again, thank you, Robert, for your suggestion.