This does sound like it could be Marek's. I have Marek's in my flock and have had symptoms come on very suddenly. Some birds suddenly start walking with wings dropped and/or are unable to use their legs. It also may be possible that he sustained some type of trauma and has a fracture, tear or dislocation.
If you want to try to save her, it is important to separate her from the flock and put her in a cage or space out of the elements. It will be important to keep her dry and safe from predators. Look at and feel her legs carefully. The legs should feel the same and there should be no excess heat, swelling or instability. If it feels like there is a problem with her leg, she MUST get vet care immediately. If not, offer her food and water 4 or more times throughout the day and change her bedding regularly. Birds with Marek's will often be interested in eating and drinking, they just may need help doing it. Try moving the dish in different spots or propping her up so she can get to the food and water. Birds with Marek's may have temporary vision problems in addition to paralyzed limbs, so it is important to really encourage them to eat and make sure they can find their food & water. IF it is Marek's, many birds will recover with supportive care. If you are able to get a vet to examine and diagnose her, that would be best.
If you suspect you have Marek's in your flock you need to be careful not to spread it to other people's chickens. Don't visit other flocks without changing your boots and/or clothes if you have handled your birds while wearing them. Also, be sure to notify people of your suspicions if you ever give away chickens.
Again, it would be best to see a vet in any case, just sharing what I learned from past experience.