Thanks for your response.

I do already have backups of a couple save games. I backup the files every so often, usually after some accomplishment such as a puzzle is completed or before I purchase a tech level (in case I change my mind).

So I already tried reverting to an earlier save game. Didn't work. I had thought of reinstalling the game. But I was hoping it wouldn't come to that. I guess I'll wait a little longer and see what ideas others might have, before I go that route.

Thanks again.