You could try gently lifting the egg in the same position you found it and place it in the nest once she leaves the nest to take a break. However it is unlikely that it is viable as swans will kick out bad eggs. Female swans also can count so she may know something is amiss from when she left the nest. If the egg has been sitting on the ground for more than an hour without protection from direct sunlight or very cold weather it is extremely unlikely that it is viable. Not always but usually the first egg layed is a dud. The Regal Swan