I had one problem like that, but it was some years ago, then my chikens lived in one cold shed, and there was a frost, one chiken slept in very cold concrete floor, she was wet (legs was wet because she plopped into the drinker one time and after that she couldt not stand or walk.
She lived two or three weeks in my house, sit near of pipes of house heating, after that she begin to walk normally. After some times i built a good wooden floor on this concrete floor, and never had this problem again.
But now it`s not winter, it`s now not so cold now... She did not fall from anywhere, or, maybe, any adult big cock standed on this chiken? I had one problem when my bashkirian drake (it`s heavy big ducks) wanted to breed width indian runner ducks (light small ducks), and after that this ducks could not stand some hours (the drake was too heavy). After this i get some strange ducks from eggs - they were something in between of the two breeds.
There are also problems with paws when inbreeding takes place, but the chicken in the photo clearly does not have any inbreeding, because such chickens usually have pronounced crooked fingers, this is always visible.
I have no more ideas...