It is a form of wry neck called opisthonotos. Wry neck is a neurological symptom, and the head can be twisted to one side, downward, or up like the picture. Wry neck can be seen in vitamin E or thiamine (vitamin B1) deficiency, after a head injury or brain problem, and in certain diseases, such as Mareks, fowl cholera, and others.
The best way to treat it is to separate the chicken to a quiet area. Give 400 IU of vitamin E, along with a chicken vitamin or vitamin B complex for some thiamine. A small bit of cooked egg, tuna, or sunflower seed would add selenium if the chick vitamin does not contain any. Selenium is only needed in a small amount to help with vitamin E uptake. Too much is not good.
Is there any imbalance or weakness of either leg or wings? Was the chicken vaccinated for Mareks disease? Videos posted to YouTube with a link here of walking would be helpful.