Our advice would be go to capture the young swan and get it to a licensed wildlife/waterfowl rehabilitation center. If the parents have banished the cygnet, then it means that they know something is wrong with it and will not try to help. If the lake is freezing, this is going to complicate the swan's ability to enter and exit the water between the ice which means it will either drown, starve or be killed by a predator going onto the ice. Good luck and let us know how this transpires. The Regal Swan