It turned out not to be an *update* problem, but an issue due to restarting the computer. In an effort to remove three different English keyboard layouts, I removed all of them. This resulted in a default English-US layout and an additional Thai keyboard layout. I seldom restart my computer for weeks at a time (I'm always in the middle of something). But when the update forced my computer to restart, the English keyboard no longer existed in the configuration; so I couldn't access it in the language bar (there was no language bar because there was only one keyboard layout installed).
I didn't make a password reset file on a USB drive, and restoring the system didn't work either. but I was able to plug in a USB keyboard and then type in my password using ASCII codes (Alt 0xxx) on the numberpad. After adding an English keyboard layout, I was able to select the appropriate keyboard before logging in each time.
I suppose it's a fairly unusual situation, but Windows should have a way to switch to a basic/standard ASCII keyboard for diagnostic or login purposes.