Thanks Carl and David!
It was a moment of great panic, and I have been very very ill.
I did work it out. It just needed to be reinstalled. The source code has finally been released, and it seems that there will be a long afterlife for Eudora now.
Yes, it is always worked with every version of windows including 10; there have been some annoying hiccups with Gmail, but Google has actually worked to resolve those; and they have simply disappeared.
All my 25 plus years of mail remains perfectly organized and quickly searchable.