I worried there would be a gloomy assessment — perhaps I would need another keyboard or even another machine.
But no, it was good news. All that was required was to flip off the key top and replace the pad underneath. "A piece of dirt may have gotten in there," the technician said. (Really?! A piece of dirt in my house?) And apparently the machine is so delicately calibrated that even a minuscule crumb can bend the little tabs that hold this responsive pad in place beneath the key.
I shudder to think of what this means for the future. I mean, I'm careful with my computer, but I can't use it in a vacuum. But I was lucky this time. The return was repaired in 10 minutes and I was once more back to fluid typing — which, like so many other things in life, one fails to appreciate until it goes away.