besides sitting hunched up what other symptoms are there? Is she snotty?, bubbly eyed?, rattly chest?, poohing normally?, is she laying ok?, does she have a temperature?, is she moulting?, what is the condition of her comb? what is her crop like?, is she walking ok? when was she last wormed? have you checked for mites and lice? have you checked the coop for mites and lice?
Sorry for so many questions but without seeing her and not knowing your girl we would need more details to even guess at what could be ailing her