Thanks for your replies. What has happened is I shut down my computer on the night I posted the problem - then restarted the next morning (as I always do), and - magically - MS Office started to work normally: Excel, Word, Powerpoint, and Visio. I was sure I had rebooted on the day of the problem - before reaching out for help - but, obviously, an overnight power off/on seems to have fixed the problem.
Thanks again for your suggestions.