Originally Posted by syg00
Normally there is a blank screen just after that splash screen that gives you the boot option. I meant in that blank screen period.
OK, I tried holding down Shift in that period, and it did indeed get me to a boot screen. Oddly enough the graphics in the boot screen were for Debian rather than Ubuntu; I guess no one has gotten around to changing that since the fork.
In any case the boot screen offered a list of operating systems to boot into. However the only options listed were two different versions of memtest. Ubuntu wasn't listed at all, which I suppose could explain why the system couldn't boot into it.
At that point I decided to stop fooling around (the machine has been idle for many days now). I booted from a live USB and offloaded any really critical files, then I simply re-installed from the live USB. Fortunately I had planned ahead when I did the first install on this machine: I created separate / and /home partitions, with all my data on /home. So I re-installed on the root partition and when I brought it back up all of my data was still intact.
I'd been hoping for a more elegant solution, not only for myself but for anyone else who has this problem in the future but doesn't have a separate root partition. However the brute force approach worked fine in my case. So I'll mark this one SOLVED even though it wasn't pretty.