If it isn't an injury to the leg, or legs, it could be a nutritional problem.
Encephalomalacia is a nutritional disorder that can be seen in chicks up to 8 weeks in age or more. I suspect this by your description. Chicks try to walk and fall forward or backward, sometimes they go in circles. Sometimes the head hangs forward, backward, or to the side. During later stages they lay on their sides with legs outstretched, toes flexed and there can be trembling.
Spoiled feed, nutritionally deficient feed, too much cod liver oil or soybean oil can cause this. First, you should separate the chick so the others don't target it for being weak. Put the chick in a comfortable, warm environment so it won't get chilled. Supply feed and treated water as I'm about to describe. Most feed stores carry Poultry Nutri-Drench. It is a fast absorbing oral remedy with A,D,E and additional nutrients. Here's what it looks like:
http://www.bovidr.com/poultry.html
Supplement water with vitamins-mineral powder mixing a fresh solution each day until symptoms disappear. If no damage is done to the bird's brain, it should recover. Supplementing vitamins in water for the rest of the flock would be beneficial too. I would suggest changing feed. What are you currently feeding them?
Here's some more info on nutritional deficiencies:
http://www.merckmanuals.com/vet/pou..._poultry/vitamin_deficiencies_in_poultry.html
