The parents may have walked the cygnets to another pond especially if your area had limited food or shelter resources or the presence of predators. They still could be in an area you can't see, so keep an eye out for the next few days.

If a predator attacked the cygnets, the parents could have gotten scared and flown away.

Another possibility is that a "volunteer" for the USDA or your local state wildlife/home owner's association killed both the parents and cygnets. Due to lack of any scientific research; these entities are trying to eradicate the Mute Swans throughout the U.S and Canada. By removing one species, the goal is to introduce the larger Trumpeter Swans for Trophy Waterfowl purposes, thus increasing wildlife budgets through hunting permits. Let's hope this is not the case in your pond, but these entities send out the "volunteers" at night to destroy nests and swans so that the public will not see anything. This at the expense of the American taxpayer. The Regal Swan