If the parents left without the cygnet it may mean that the cygnet cannot fly or something may be wrong with it. If the parents know something is wrong they may abandon it. If there was a predator or something else scared the parents, they may have fled the area.

There is also the possibility that something happened to the parents.

In your, state wildlife officials are culling Mute Swans meaning they are killing as many as possible. If you contact wildlife facilities about the young bird they may kill it so you have to check around for a no kill shelter. If the cygnet cannot fly, it may not be able to survive the cold winter, a frozen pond or predators. Therefore it may need to be brought inside at night or during the winter. If you can capture it and you cannot find help, you might try to contact Bob Knox at Knox Swans in Illinois. The Regal Swan