SSH/PuTTY says "Server refused our key"

When logging in via SSH/PuTTY, the console first asks "login as:". I put in my login and obtain the console message "Server refused our key". PuTTY also provided a popup message: "No supported authentication methods available (server sent: publickey)"

There are a number of things that could have caused this. Sequence of events:
(1) In SSH, I launched the process to upgrade from 20.04 to 22.04 in place.
(2) On one of the dialogue screens that came up, I stupidly attempted to copy using a <ctrl-c>, rather than just hitting the offered "OK". I think this was unexpected input as the program immediately went forward without my hitting Enter on the OK and displayed errors.
(3) I went through several cycles of exiting, rebooting and apt-get updating/upgrading, because when I re-entered via SSH I was told I needed several hundred programs updated.</ctrl-c>

It was after a few iterations of (3) that the issue with SSH access not working arose.

My SSH makes use of Pageant.

LISH access works. My email server and Jetty webserver, and a couple SpringBoot applications all continue to function but I have lost HTTPS (secure server) functionality, can only go to websites with HTTP. (I have been using Certbot with success up to this event.)

I would appreciate advice on what to investigate/do in order to be able to log in again via PuTTY using the local RSA key.