My list of things to check if you dont have a clue whats wrong.
Check for mites and lice
Check crop for impaction or sour crop
Check for egg bound or internal laying
Check mouth and vent for anything unusual
Check poop for normalcy or parasites
Check eyes and nostrils for discharge.
Give warm bath and if the crop is good give something warm to eat( cooked oatmeal, cooked eggs or yogurt) and vitamin/electrolye water (or a little sugar water in a pinch for energy). If everything checks out ok on the list and the bath and water dont work then I might consider worming after a couple of days of trying to build up system.