It may have become separated from its parents. There are only four reasons that a cygnet will get separated:

1. It wondered off even though mother swan tells it to stick close.

2. There is something wrong with the cygnet and the parents have banished it from the rest of the family so that predators cannot find the sick or injured bird and it leads the predator back to the rest of the family. Or, the bird is sick and the parents do not want it to infect the rest of the family.

3. Something happened to the parents.

4. The cygnet is too young to fly, may have been the runt of the family and its wings have not fully developed to fly with the rest of the family.

If nothing happened to the family and there is nothing wrong with the cygnet, then give it a day and the parents should return to the cygnet. However, if the parents do not return in a timely manner and the cygnet cannot withstand severe inclement weather, protect itself from predators, find food or generally take care of itself, you might try to rescue it and take it to a no-kill wildlife/waterfowl rescue/rehabilitation center. If there is nothing wrong, other than it is underdeveloped to fly, it can overwinter in the shelter and then be released back onto the pond in the spring. Then, the young swan should be able to keep up with the rest of the flock during the next migration. The Regal Swan