Mystery illuminated .... if not fully resolved. What happened is that the Run/Debug configuration was deleted. How could that happen? This link explains a "Known Issue" that will "remove run configuration information" from projects opened with AS Ladybug.

Run configuration information removed

I have suspicions that later AS versions still exhibit the issue. I was using Meerkat, but I can't be sure that version caused the problem. View the link for the background information.

MAKE SURE YOUR VCS IS WORKING. You will have to restore your project. (I learned the hard way.)