She appears to be tipped forward, so the first thing I would do is check her crop? Is it full or empty? Large or small? Hard or soft? Compare to other healthy chickens. Check it at night, remove access to food and water and check it again first thing in the morning. If it is not emptying overnight then that suggests a blockage/impaction.
What does her body condition feel like, particularly around her breast bone.... does she feel reasonably well padded or skin and bone?
Check her abdomen below her vent and between her legs for any swelling and her vent itself. Is there anything unusual? What does her poop look like? Does her vent look like it is pulsing?
I'm a little concerned that her body may be contorted due to Marek's disease. I haven't seen one hunch up like that before but it is a very unnatural posture and Marek's can cause a whole range of unusual body contortions. How old is she and how long have you had her? Have you added any new birds to the flock in the past 3 months and if so, where did you get them from?
How does she walk? Can you post a video of her moving? It would need o be downloaded to the likes of You Tube and then a link posted here on the thread.