Probably from sitting too long on eggs. My Big Broody BO was like that when she hatched eggs. She was so bad that she couldn't get back up to her eggs and I had to lift her back up to her nest box. Somebody may tell you different but I would give her a box on the floor for her chicks and her and fence the other birds out so they have a small place to be a family. You don't want her trying to fly down from anything while she is in this condition. If she can't walk the landings will become crashes and she could hurt herself. I bet she will be OK in a few days after using her legs more. The Little Crippled BO is another story. Still a mystery what happened to her but I still have her. She can walk good now but I almost put her down. I think she got hurt somehow. She spent many weeks confined to a nest box because she couldn't walk at all. I put a piece of hardware cloth over the front of the nest box so the other birds could not get to her or her food. Took her out twice a day to clean out the box. She was so bad she would roll over on her side when I sat her on the floor of the coop. I used straw to form a deep depression inside the nest box to keep her upright and from rolling over. Fed her lots of fruit and treats to keep her eating. Kept water in a heavy bowl directly in front of her and lots of food. One day she stood up and then I had hope for her. It was a slow process but she finally got where she could stand and eventually walk a few steps. It seemed like forever but she finally rejoined her flock. To this day she still roost on the floor and has never roosted with the other birds. She has a board that is about 3 inches higher than the coop floor and she stands on that all night. Never dreamed she would make it but she did.
we did not, we had a hen seating and took part of the coop fenced off with a door to get in, then we put a large box with fence on top had three sides with open side at the little door so if and when she wanted to go out she could, we just put chips on floor and she was a good mother, never saw anything like her, one day she was walking back and forth in front of big door so I opened it and she came out, later I went out and she was at the gate I opened it and she went in all the peeps came running to her, she had 13, 2 days later she was walking back and forth again so I opened the door and she moved to the door and turned around talking to her babies for a min then she walked out closed door and she never went back in, I never saw a hen talk to her babies like she did and we have had chickens for 5yrs. the cage works great