She may have suffered a bad injury affecting her spine, but it would be hard to rule out Mareks disease without getting a necropsy. You could try her in a chicken sling with her feet touching the floor for periods throughout the day, and have food and water within reach. Give chick vitamins...
Get some chick vitamins with electrolytes, and try to get her drinking. She may just be dehydrated or overheated. Is she having any runny droppings? While Mareks may be a possibility, even coccidiosis can cause weakness or imbalance. Eating moldy feed or botulism can be other causes for...