While I was out of the room, my screen went from 17" wide to about 13.
I can reduce text size to see OK, but can't figure out how to get back to full width.
I see nothing in the control panel or in Help under 'screen width.'
Help, I'm sure it's basic.

What version of Windows do you have?

8.

Sorry, can't help you. Google the issue, chances are you'll find others who had the problem and offer a solution.

If you have got to the control panel, you need to look for screen resolution and play about with that.
It sounds like it may well of reset to a non-wide screen res without stretching to fit the panel.

Curleyben, that was it! THANKS!

(Leaving the room with the computer on + a cat in the house is always a mistake.)