tried restarting your computer? It fixes multitudes of issues that have not explanation.

Thank you, Matt. Yes, I not only turned off the computer but unplugged it (superstitious, I guess). When I rebooted, it was working again and the Thunderbird icon had reappeared on the Windows 10 app list. I suspect that the program hung up during the recent update, and restarting it completed the installation. Now if I could only figure out why it constantly hangs up.

The most common cause of failed updates and to some extent hangs is in the anti virus program trying to either scan the update process, or block the updated version as "unknown"

That's so interesting! Now I believe I will be listening to hear the anti-virus program "thinking" before it allows an email to load or open. (You can tell I am a fiction writer, not a computer person.)  :-) Thanks again.