5 week old chick can't walk! Help!

I received 15 chicks (day old) from Ideal 5 weeks ago. They have been completely healthy and normal for 5 weeks but now one of my orpingtons can't walk. She can stand and grip my fingers with her feet but she falls over when she tries to walk. She was normal a couple of days ago. What could be wrong with her and what should I do? She is still eating and drinking but am having to bring the food and water to her. The chicks are on a chick starter/grower diet with 20% protein. It is commercial but is not medicated. The other chicks look fine though they look a little "rough". I'm thinking the roughness is just because they are growing chicks. What do you all think and what can I do? Please help fast!