Lots of things can cause a bird to sit there lethargically....worms, internal bacterial or viral infection, egg binding, injuries from other birds or hitting their heads on the ceiling, internal ailments of all kinds, coccidiosis, bad diet...the list is long.
So the best thing you can do is go over your birds area, give them an exam checking for mites, lice, egg binding, injuries, if you have never wormed and they are more than 6 months old consider worming, if they have any signs of respiratory ailments, get them on antibiotics. Check the feet to make sure they don't have bumblefoot (infection in the pads) If they are acting fluffed up in the corner, runny poop or blood in the poop they could have cocci. You would need to add Corid to the water for 5 days. Check the poop for color. Ashy grey looking poop can be a form of enteritis to which you would want to add antibiotics. Make sure they are not being bullied, have fair access to food and water. Keep them in a clean environment, give them something to dust bathe in. If you suspect egg binding, give them some crushed up Tums and soak the lower half of the bird in warm water for 10 mins to help move the egg. Always offer up crushed oyster shell for the layers.
Just a few ideas. If you can find some other symptoms, we can go further with the healing.
