Neither of the above two answers has solved the problem. Maybe I can move the ball forward a couple of inches ...
I, too, clicked on the wrong resolution for my external monitor and now I can only use the monitor in "Mirror Displays" mode. When in separate mode, the external monitor is black except for a message saying "DVI NO SIGNAL". I cannot see the System Preferences > Displays control dialog for that monitor because it's on the black screen. When I return to Mirror Displays mode, I can see that the wrong resolution is set and when I set the proper one, I get both monitors work -- but ONLY if I stay in Mirror Displays mode.
This sounds like exactly the problem that @CyberSkull reported, same cause, same results.
I can verify that
Colorsync Utility.app does nothing for resolution, only color profile, and my profile is likewise set to Factory Default.
Similarly, the answer about
hmscreen is about Arrangement, not Resolution. I looked at the doc for
hmscreen and it confirms that it deals with ScreenIDs, screen positions, and which screen is the main screen, not resolution. I'm afraid to try it to set the black external monitor to be my main screen, and that seems to be the only experiment it might be useful for.
Finally, I found a solution that worked for me! In another Ask Different question, "change screen resolution with AppleScript", more than one AppleScript solution was offered (which I didn't try) AND @user332214 posted a link to a utility developed at the University of Utah, Display Manager which worked for me.
Display Manager supposedly has a GUI interface available, but I couldn't find where it had been installed to invoke it, so I used the command line interface. Once it was installed, the command line:
display_manager.py res 1920 1200 ext0
successfully changed my external monitor to 1920x1200 resolution, my usual, from the erroneous 1920x1080 that I had accidentally set and which resulted in a black screen except in Mirror Displays mode.