More important detail I should include:
After spending a few minutes trying to figure out what was wrong, I restarted the 'tor' relay daemon. It came up like this:
Bootstrapped 5%: Connecting to directory server. We now have enough directory information to build circuits. Bootstrapped 80%: Connecting to the Tor network. Bootstrapped 85%: Finishing handshake with first hop. 2 entries in guards Failed to find node for hop 0 of our path. Discarding this circuit. 2 entries in guards Failed to find node for hop 0 of our path. Discarding this circuit. Self-testing indicates your ORPort is reachable from the outside. Excellent. Publishing server descriptor. Failed to find node for hop 0 of our path. Discarding this circuit. Failed to find node for hop 0 of our path. Discarding this circuit. . . .
This says to me that much or all of the Tor network was contactable at the time of the restart, _but_ the two entry nodes were not reachable.
My next action was to stop the relay, comment out the 'EntryNodes' line, and start it again. Came all the way up immediately. Interestingly the two favorite entry guards (still present in the 'state' file) were now accessible again and were retained as the chosen guards. I put back the hard-EntryNodes config and restarted and again it came up fine.
The restart where the entry guards were not contactable is what really made me wonder what has happened here.
I did notice in the 'state' file a line indicating each guard was "down since xxxx". I take this as simply informational and that it would not have prevented the relay from attempting to reach them when it was restarted.