Is her breathing labored? Does she eat and/or drink? Have you tried syringing water and/or egg yolk or moistened feed into her? (have to do this carefully so it doesn't go into her lungs, there's directions on this site) Have you checked to see if she has any physical injuries? Sounds like she may have broken her neck or have a debilitating respiratory illness (some of those kill within a day or two, sudden collapse is one of the signs). Are any other chickens sick or showing symptoms of any kind? Does she move other parts of her body at all, or only breathes?
