I can hardly believe it, but I solved it.
The clue was in that long printout from the Windows troubleshooter:
Windows cannot connect to "[my WiFi name]"
The wireless network security key is not correct.
I spent a lot of time trying to figure out how to correct the network security key. Somewhere along the line, I figured I would uninstall the network adapter. I had reverted to the previous version, and when that didn't work, updated it, but I realized I had never uninstalled it.
It took me a while to figure out how to reinstall it. Eventually, I used the Dell process of detecting my computer and then telling me which drivers were recommended for my system.
I installed two of the top 3 - one of them, an Intel driver, would not install. I am pretty sure this is the one that fixed the problem:
I did have to reboot, and then I right-clicked on the WiFi icon in the system tray, selected my WiFi, clicked "Connect", and then it asked me for my network security key. Fortunately, my ISP, which provided the router, makes it easy to find the security key (sticker on the router). I typed it in, and lo and behold, I had WiFi again.
I sure hope Microsoft fixes this problem so that more people do not have to spend nearly 14 hours searching for a solution!