Thanks, tacit; the problem apparently was the lack of a startup disk selection, but it didn't occur to me to look (although it should have
).
It's not something I remember ever having happened after an update, and OS related issues have become so few and far between that I've forgotten most of "Troubleshooting 101".
This was a strange update: after clicking on "Update" I got no indication that anything was happening until my screen suddenly went black after about 15 seconds.
