Mac Mini won't boot after chime
When I power the late-2012 Mini on, I hear the chime, then . . . nothing. I've tried booting with CMD-R to get to Recovery, CMD-Option-R and Shift-CMD-Option-R to get to Internet Recovery, and Option to get to startup manager (even if I could get to Startup Manager, I have no external disk with macOS installed). With all of those keyboard options, the screen flashes "Entering Standby" and goes black.
The keyboard is the Apple bluetooth model that came with the machine. Batteries are new. The computer was functioning perfectly normally on its most recent previous boot.
Would this total failure to boot into normal mode or ANY optional mode suggest that the motherboard is faulty?
iMac with Retina 5K display, macOS High Sierra (10.13.5)