Is it a baby duck.. Aka duckling? If yes then the first answer is right. If its an older duck and its spring or fall then it is molting. Just like chickens. It should not loose all its feathers to the point of Bald. Some wild drake ducks (males) will molt into eclipse feathers. Feathers just like it female mate! Why? The bright flashy mating colors make it hard to hide from predators. So they go away when not needed.
Baby ducks loose their fluffy feathers so the can grow feathers that are better to swim with....older ducks feathers do not hold water, making it easier for them to glide across the water
Nobody has mentioned that ducks will go into a forced molt if under stress like after an injury, attack etc...