Thank you so much, @Philippe and @stephematician. Most of the commands suggested simply weren't there. And some, such as reboot, did nothing but return to the initramfs prompt.
I was also unable to get my phone connected to provide access to the internet, so in the
end I decided to make a clean start. I had backed up all of my home folder on Monday, and as this happened about 1:00 PM Tuesday, there was nothing critical lost.
I downloaded the UM 24.04.2 iso on another machine and put it on a USB stick. But the installation failed. The install dialog just vanished. Tried that again, same result.
I still had a USB stick with UM 22.04, so I installed that. Smooth, efficient, no worries. Then I used Software Updater to bring that version up-to-date. When it advised me that the software on my computer was up-to-date, I clicked on the button to upgrade on-line to UM 24.04. The on-line upgrade went smoothly.
I tweaked the desktop, restored my backed-up home directory, loaded a few programs that were lost (Inkscape, Brave, etc.), and everything is back to normal.
Well, almost normal. There's an annoying problem with Onboard. If I can't iron that out myself, I'll start another thread.
Again, thank you both so much for your help.