No problem, I thought -- unplug the screen and then plug it back in, and this would reset things and all would be well. But no! The laptop remembered the screen and set it straight back to the undisplayable resolution. Setting it to mirrored and back also did not clear it; neither did a reboot.
To cut the story short ... go to the Displays System Prefs panel and click on the option to show the monitors in the menu bar. Now you can go to the menu bar (on the functional laptop display) and reset the frequency/resolution for the second screen back to something displayable.

