We did have two nights of very cold temps, and heavy rains and a thunderstorm the night before the cygnet died. That seems like a very strong possibility. I also had noticed that the adults wouldn't allow one of the babies to climb on their back, but did allow the other one to climb aboard, so maybe they sensed it was sick.
Thank you for responding so promptly....we were heartbroken, but are praying that the other cygnet survives.