I did nothing
Every now and then, "automatic" features of technology decide not to work... and I never understand why. I checked the site last night, and I couldn't find a problem, and manually running the AutoSSL update routine did not fix the issue (it should have). The only thing Internet searches revealed is that other sites had had sporadic problems in the past with the same service. I submitted a support ticket to our host and went to bed, and ten minutes later when I checked the site from my tablet, it was working
Bonsai and web site maintenance both teach patience
I could have started breaking things last night trying to fix a problem that wasn't broken (at least on our side).