There was a construction delay!
While I suspect I might have given up -- or at least stopped for lunch-- the bird who was standing on the nest when it collapsed flew off and returned a moment later with a new branch. In the meantime, the partner flew up, looked around for a second but added another branch and they kept working. I did not hear a chirp or squawk that could have been interpreted as "what did you do?", "I just put that piece there!" or even "I told you that mud was too soft!"
They just kept taking turns, quietly rebuilding the nest.