Slipped tendon?

My 4wk one lavendar Orpington began laying down more than usual about three days ago. Still up to eat and drink. Two days ago I noticed her started to limp. Yesterday she could not put weight on her left leg and was starting to get trampled by the rest of the chicks. I separated her and one other of her companion chicks into a separate brooder last night. She made it through the night and drinks water when I dip her beak, and seems to be eating the food I put in front of her. But she won’t walk and her feathers are very puffed up and dirty looking. She can put weight on her legs when I stand her up, but her left leg will splay out backwards or slightly to the side.

What is this? What can I do to help her?