I had a similar situation in my backyard last week. For no known reason our six month old Olive Egger, Whoopie, Could not stand up without falling to the left side. Our other three chickens were not affected.
All my research online turned up horrible pathologies. We went to bed the next night praying that nature would swiftly take her course. The next morning I found Whoopie in the middle of the backyard with her head turned 180°, sitting in the rain. I wrapped her in a towel and began to massage her neck. She had torticollis for at least 36 hours. She responded to the massage, and I found a definite right lateral bony mass, a subluxation in her upper neck.
I manipulated her neck by toggling it back and forth. She seemed to get better. I did more that night and the next morning. By the third day she was back with her sisters, chasing bugs and living the good life.
I'm a chiropractor by profession, go figure. But, I was disturbed that none of the chicken pathology sites suggested subluxation. Then again, most MDs are ignorant of this problem, as are most Vets. Just too simple for them, I guess. Also, there is no money to be made in medications.
I have some footage of my manipulation on my clinic Facebook site; KenmoreChiro. Feel free to check it out.