There could be any number of reasons for her death to include, egg impaction (meaning she did not completely remove any or all eggs inside), illness from sitting so long on the nest (she can expend about 30% of energy from laying eggs and nesting), the older male not intentionally, but because of age may have hurt her internally while mating and she was not old enough for this process. In this case, the internal injuries and/or infection could have killed her. It is really too difficult to determine death without a necropsy. Is the male watching the young swans? If not, you will need to get them to a waterfowl/wildlife rehabilitation cetner to help raise them. The Regal Swan