5 Week Old Chick has Limp and Useless Leg?

[xcchickenman]

Hey everyone! I recently bought 6 new Golden Comet chicks from my local Tractor Supply. They have all settled in nicely; however, the other day I found one chick lying in the yard, unable to move. One of her legs is limp and she doesn't seem to be able to move it. All she can do is stand on one leg or lie down. I let my chicks run out in the dirt during the day when I'm home and its warm enough, and then bring them at night to the heat, so on the day I found her she was in the middle of my garden. She didn't appear to have been attacked. Could it be a disease or some other injury and how can I fix this?

I already have her separated from the others and in a shoebox with food and water!

Thanks!