How old is she and was she vaccinated for Marek's Disease? If she is under a year old and there is no sign of obvious injury or infection, then the commonest cause of such lameness would be Marek's Disease. Even if she has been vaccinated for it, it is still possible.
I give my Marek's birds a good quality vitamin supplement, keep them well fed and give them access to sunshine and grass in a safe environment whenever possible. If it is Marek's she will be actively shedding the virus whilst she is symptomatic. Your remaining flock have almost certainly already been exposed (there is a minimum 3 week period of incubation for the virus before symptoms can exhibit) but it is probably wise to keep her isolated from them to reduce the build up of infected material in their environment.
You may find that she recovers quite miraculously and as suddenly as she became lame but if it is Marek's she will be prone to future outbreaks, weeks, months or even years later. She is not infectious during the dormant phases of the disease in the same way as people with the cold sore virus are not infectious when they don't have a cold sore..... both are caused by Herpes viruses, so she can return to the flock then. Keeping her happy and stress free and well fed will be key to helping her fight it. I try to ensure they can see the flock (downwind as the virus is shed via dander dust which is inhaled to infect other birds) for at least part of the day, but in her own safe enclosure. There is hope as long as she remains bright eyed and alert like that and keen to eat.
Good luck with her.