Need advice. I have a sick 4 month old Rhode Island Red!

Just went out to feed my girls and 1 was lying on the ground. She is very alert making clucking sounds like she normally does when I pick her up but she is unable to sit up or stand. They were all fine when I let them run yesterday. They are completely enclosed and we have an outside dog that keeps away other animals. The dog loves the chickens and never bothers them and is unable to get near them when they are put away at night so I know dog attack isn't to blame. I brought her inside to see if some food, water and warmth might help. Does anyone know what may have made her so sick so fast? They are let out into the yard everyday and the others are fine today. What can I do for her? Thanks!