Hi Caroway. It sounds like what is called 'star gazing'. It is a neurological problem apparently, which we have had in an Orpington hen one Summer brought about by heat. Suppose it is the chicken equivalent of heatstroke.
I've read that a course of anti-inflammatories can help. In our case it happened very quickly, literally before our eyes. The hen was so distressed and in danger of injuring herself she was despatched. In retrospect we should have put her in a cold place in complete darkness to see if the condition improved. She was disorientated because her neck was twisted backwards and round so she was looking backwards and upside-down. Strangely, when she was offered a treat the condition corrected to grab it but then returned immediately. That now says to me it should be curable.