Things were broken, and now they are not.
This week WordPress 5.5 "Eckstine" was released. I upgraded my website, and things immediately went sideways. But the problems started earlier, on 6 August, after I upgraded to JetPack version 8.8.1.
I didn't suspect JetPack 8.8.1 as the cause at the time, but my website was slower, and unknown to me, the upgrade broke the connection between VaultPress and my site was broken. JetPack is required to use VaultPress properly. Still in the dark on the issue, on 11 August, I upgraded to WordPress 5.5, and everything broke. My website went offline until 15 August when I recovered manually from a backup I downloaded from VaultPress.
I want to express my gratitude to the web designer and WordPress consultant Daniel Brinneman for his help in brainstorming avenues of troubleshooting.
At some point, after I calm down, I'll write up a longer post on what I think happened, what I did to recover the website, and lessons learned.
Submitted as part of the 100DaysToOffload project.