Thank you everyone. I have judiciously followed everyone's advice but to no avail. However, we are now up and running.

Remember that all was working until Windows imposed an update. After that, no external display. Because everything had been working and no hardware had been touched, I assumed that Windows was guilty (as it often is).

In the past, I've been bitten by the assumption that (not motorized) hardware does not simply fail, and I may have been bitten again.

Finally, is desperation, I replaced the HDMI cable with an RGB cable and voila, all is well. Whether the HDMI port on the computer has failed or something in software is messed up, I have no idea but at least she is up and running.

Hopefully my frustrations will help someone else. Thanks everyone for your suggestions.