Hi cmyiyc,
Thanks so much for your response.
I am using genuine Arduinos.
In the end, I checked with two more cords, more Arduinos, and they still didn't work. I then checked the System Report. It did not show the Arduino devices on the list. Not knowing how to reload the firmware, I realized that I hadn't yet tried to plug in the Arduino I used when everything was working last. Presto! It worked. And then all the others have worked after that. I still have no idea what the problem was, but perhaps it doesn't matter what the issue was if it's now solved.
Thanks again for your advice!
Tori