This sounds as if a predator (human or wild) attacked the nest and destroyed it. If this was the case, the parents would have escaped with their lives and left the area. They will not come back to such a scenario as nature requires them to keep themselves safe and healthy for perpetuating future generations. They will not risk going back to such a disturbance as unhatched eggs are not as important in nature as the parents' lives.

We are not sure where you might be located in the U.K., but would highly suggest that you contact the RSPCA or Swan Support in Datchet to alert them to this situation. The last thing that anyone wants is to have human interference in swan nests as it is not only illegal but extremely inhumane. If a human did cause this situation, then an investigation and possible prosecution must be undertaken to ensure it does not happen again. The Regal Swan