A little background. I have both gnome and KDE installed in the particular computer. The home WiFi network is setup in both. In KDE, it is not set to automatic connect. In Gnome, it is set to automatic connect, and it is set to “share with other users”. The effect is that the connection definition file is in “/etc/NetworkManager/system-connections”, and NetworkManager connects before any login. This has been working for a while.
Most recent use, before today, was yesterday afternoon. NetworkManager performed perfectly.
Today, I booted it up. I saw the 30 second countdown of NetworkManager trying to connect. Then booting completed. I logged into KDE, but there was no connection. I had no difficulty connecting within KDE. I checked router logs, and they did not show any previous attempt to connect.
I logged out of KDE and into gnome. Now there was no connection. I went into the network settings (which requires the root password for a system wide connection). There was no network key showing for the home WiFi connection. Manually checking the definition file from a root xterm showed that indeed, the network key was missing.
I reconfigured the key, and the network began working as it should.
Somehow, for no obvious reason that I can see, the network key was deleted from the network definition file. This is presumably a bug in NetworkManager, though probably a hard bug to reproduce.