Hi: I don't know where you live, but I know a friend has lost all but a few of her chickens to fox predation. We live in a semi-rural area, with the occasional fox and lots of skunks, dogs (some wild, some not) running around, sometimes coyotes. My friend found a hen who'd been badly "dogged" in an irrigation ditch. The white hen, whom we named Blanca, had her back feathers ripped to the flesh and was in very bad shape. The friend treated her with antibiotics and, after about four months, Blanca grew back all her feathers and was the queen of the yard. Unfortunately, this past spring Blanca began to stay outside at night (it's New Mexico and we get very warm here) and was probably killed--along with most of her hen pals--by a fox. The fox will bit the backside of a chicken to weaken it. One of my friend's other chickens had its tail feathers pulled out, but survived.
Because of our situation and all the predators--domestic and otherwise--we've built a secure roost with a floor and fence and plan on putting the chickens to bed in their Chicken Chateau every night. Should be easy, as I'll just put their feed in the coop and then close them in. It was an expensive project but, after what my friend has gone through, it is worth it to us.
Hope your chicken recovers.