When I was in high school back in the sixties in New Jersey, "Prom to Mom" was a thing to be assiduously avoided. Just sayin'.
Picking up from the previous post, my computer remained out of service for our whole time in the Pacific Northwest. Several times I took it out of the carryon bag and tried to start it up, including attempts with the magic three-keys-plus-powerkey combination, but the display did no more than issue a dim glow visible only in subdued light. No way to enter a passcode.
When we got in Saturday evening a little before ten, I did a bunch of chores, still being sort of on Pacific time, and got the MacBook Pro out, docked it to the Thunderbolt display, opened it up, and hit the power button. I wanted to see whether perhaps it would light up the external display even if the internal remained dark. Nope. It started up exactly as it should and always has until the second day of the trip. Retina display and Thunderbolt both perfectly normal. I shut the laptop and slid it under the big display, which is where it lives most of the time. Replaced the batteries in the Bluetooth keyboard and mouse, and everything worked normally.
Next I dismounted the external hard drives docked to the big display, put the computer to sleep, and undocked it. Opened, it lit up immediately, ready for use. OK, then. I shut it down, waited a couple minutes, then hit the power button. Utterly normal startup routine, no sign of the problem that made it useless for the past ten days.
So, looking on the bright side, my computer is working again. On the other side, there's no reason to expect it to continue to work properly. It has just failed abysmally at the task for which I hired it four years ago: to be the brains of my main computer system, able to disconnect and go on the road leaving behind only the big display and external storage. I suppose it's possible that if I take it to an Apple Store and have them hook it up to hardware diagnostics they might find something out about what happened.