Around 7:15 PM this evening Debian networking went away.
I logged into the console and everything at first appeared normal but I could not ping or connect to anything outside.
I tried restarting networking, it was uneventful but did not restore connectivity. I think restarted NetworkManager but it failed to start with the error stating it had been restarted too frequently.
At this point I rebooted the machine. After it came back up there was still no networking but the NetworkManager was working again. I went in and found the Ethernet device was just gone. I recreated it and now everything is working again.
This must be a new NetPlan feature.