How old is your chicken? Was she vaccinated for Mareks disease? Could she have a head injury? It looks like wry neck or torticolis, which is a neurological symptom. Head injury, vitamin E or thiamine (B1) deficiency, or certain disease can be possible causes. Those include Mareks and some rarer diseases such as avian influenza and Newcastles. Where are you located?
Most people give 400 IU of vitamin E and a source of thiamine, such as 1/4-1/2 tablet of vitamin B complex daily for a couple of weeks to see if symptoms improve. The chicken may need to be hand fed if she stops eating and drinking. Hopefully, her symptoms will improve. Watch to see if she needs protection from others, since they sometimes get attacked when acting unusually. Here is a video of feeding a chicken with wry neck: