By total serendipity everything seems to be working.
I do not recommend this to anybody but maybe to somebody with more knowledge they may gain some understanding.
From the Apple support site I found and option to reinstall the OS that originally came with the computer.
It started by saying, “Turn on your Mac and immediately press and hold Command (⌘)-R until you see an Apple logo or other image.” Then there were some options to follow.
This ground away for about fifteen minutes but did not work. There was some sort of cryptic error message that ended in 2100F. I tried a second time and got the same result.
At that point I abandoned work on my current computer and dug out an old computer. After I got the old computer working and down loaded my code from iCloud I posted my problem on the board.
I saw your response and welcomed the opportunity to help. I launched terminal and started following the instructions. However, I miss read about opening Arduino through terminal and double clicked on the Arduino icon in the dock. To my total surprise it launched normally and functioned normally.
Terminal was new to me. To satisfy my curiosity I did type “open Arduino” and got this message:
“The file /Users/my-name/Arduino does not exist.
my-name-MacBook-Pro-2:~ my-name$” (I am substituting “my-name” for my real name here.)
What the failed Command (⌘)-R attempts, or something else I did, accomplished is beyond me. But I am functional again.