Sorry to hear about your bird.
Approximately how long has she been acting lethargic? Do you have a "name-brand" for your feed? Are you providing a source of calcium for her? Is she eating, and drinking? How do her feces look?
One of the most frequent problems experienced when raising waterfowl is laying problems. Often times with new layers, especially heavyweight breeds. Sometimes they'll try passing a soft-shelled egg, or an abnormally enlarged one. This is can cause them some stress, which can subsequently result in sudden lethargy, and a decreased feed intake.
Luckily the problem tends to resolve itself, but an administration of a calcium supplement such as calcium citrate, Tums, or calcium gluconate may be good to aid in the passage of the egg.
