One of the three-door switches had failed. In my case, 00631512 or ref 25217585 on the left front of the unit. The green button no longer "clicked". When that switch fails, one of the ceramic fuses located under the top cover at the far back of the unit is blown. Used Volt-Ohm meter to determine which fuse. Happy to have also ordered a replacement door switch Support, just in case, as I was unable to remove the failed switch from the support without breaking a key brittle plastic tab.
Separately, the unit also had a dim display, ultimately discovered unrelated to the loss of power. Had just prior replaced the microwave control panel & electronic control in case that was culprit, but door switch & fuse were a separate problem. I found it easier to access door switch support with the control panel re-removed.