It could be many or the combination of many things. If you had a high tide and the eggs were submerged for any length of time, the developing cygnets could have drowned. The eggs could have been infertile from the beginning or they could have been ruined by intermittent cold or hot weather if the temperature fluctuated to great extremes in such a short time. The pen could have been frightened by a predator in the area or suffered such extreme temperatures that sitting on the nest was becoming detrimental to her health, i.e., no shelter from extreme cold and wind or extreme heat and no shade. Is the male swan still in the area or did he leave? If he departed the area or left the female to solely tend the nest, this could be a reason that she left as she got tired or could not spend time to eat and bathe and could not continue the rigors of being the sole nest protector. In any case, swan parents know when something is wrong with eggs and will not spend any unnecessary time tending to eggs that have no viable chance of hatching. The Regal Swan