It is not necessarily the case that the cygnets should have hatched already. The weather patterns this year has created some early and late nesting. She may still be sitting on the nest. Have you checked the area where they usually nest to see if she is still there?

Mother swans do not begin the incubation period (in other words don't begin to count) until the last egg has been laid. Give it a couple more days to a week and see if she reappears or check the area to see if she is still nesting. The Regal Swan