Thank you for the reply. I'm using version 3.5.9 of the app.
Part of the problem that I neglected to mention is that I was unable to do anything in the app after getting the alert (it wouldn't connect via cloud), but, after some more attempts over the next couple days, I was able to connect. I then rebooted the A3 remotely, and my Nest products came back after that.
When I returned from vacation, I found clocks flashing, so I must have lost power briefly, triggering the events that lead to the problem (I assume).