Hi Lisa There could be any number of reasons but a sudden change in normal behavior or routine may not be good. The swan may be injued or ill. Is it staying in the same area? This could mean an entanglement and it needs to be immediately rescued. Are both swans males or females? One young and one old? This could be a territorial issue. Are there any predators wild or domestic in your area? The swan could have been startled or chased and is scared to come out of the water. Did a human specifically children chase or scare it? Was there anything added or taken away from your yard that would be unfamiliar to the swan? Were there any loud or new noises as from construction in the area? As you can see anything may have caused the swan to alter its behavior. Keep observing to ensure that it is eating swimming bathing and preening. If not you need to capture it and seek veterinary care. The Regal Swan