Thanks to all of you who were helpful, and I learned a lot. I took all your suggestions to heart, but oddly enough they id not prevent the same problem from happening one month later. No power failure this time. I finally got through to someone at MS who gave me a solution and a likely cause.

First the solution: Open a new doc and Insert>file the doc that won't open. New doc opens perfectly containing the old doc intact.

Likely cause: Old doc "damaged" from copying and pasting from one doc to another when the paste contains the last carriage return in the file.

When starting a new session of editing, I used to open a new doc, save with a new name and then copy the old doc and edit it. I started at the top , did control-end, copy, paste. That last carriage return contains info Word needs to open.

You may remember that I also sometimes work out what I'm going to write in one file and copy and paste it to the "final" file. Since what copied was often the last thing I write, I'd guess that I include final carriage returns at this step, as well. Hence, mega-damaged document.

The insert>file works perfectly so far for old documents, and I no longer include the last carriage return.

Maybe someone else can benefit from this.......