Been running Moi on my iMac (27" retina display) for almost two years now with no problems. Last time was about 3 weeks ago. Now this evening I launch the program and get a:

Unable to create Direct3D9 device.
Possibly an unsupported older video card, or hardware acceleration is disabled.
Check Control Panel / Display /Settings /Advanced / Troubleshoot

Message in the main window. I have not changed anything (that I am aware of) and the Mac OS has not had a "control panel" since OS 9 (well over a decade ago) and I don't see anything in the display preferences at the OS level that corresponds to a 2006 thread here int he discussions about "Unable to create Direct3D9 device." issues which discuss 32-bit color depth.

Anyone have an idea how I can fix this or even what to troubleshoot?

At this point MoI is suddenly non-functional as a result of this suddenly appearing issue.