No interest in food is not good. I would try to tempt her with anything, but if she will not eat then don't leave it too long before ending her suffering. I would give her electrolytes in her water to support her but if she is still not eating in a couple of days, then it would be kinder to put...
I have seen that in birds with Marek's disease. Not saying that's definitely what it is but she is the right age and it is a very common disease with a huge range of symptoms.
The neck becomes contorted and atrophied similar to wry neck but lower down the spine.
You can try vitamin E and foods...