Non-specific signs such as lethargy are hard to diagnose over the internet; your best bet to resolve the problem would be to see a veterinarian in your area that is experienced with birds.
Until then, consider moving the duckling into a separated enclosure near the flock with access to feed, and water. The area should be warm, and away from anything such as dogs/cats that could stress the duckling out.
If the bird is willing to drink/eat, try offering some warm electrolyte fluids, and mashed scrambled eggs. Both will help address any current (fluid, electrolyte, and nutritional) deficits, and get the bird into a more stable condtion.
In the case you cannot see a veterinarian, please post a video of the duckling, as well as any information you may think would be useful. ( diet, fecal pictures, past illness, etc)
