Not meaning to lecture - but you really should quarantine new chickens for a week or so before mixing with the flock. You never know what you're getting at a fair like that.
It's can be a pain, but worth it when you end up with a sick one.
If it's still down - I'd quarantine it even if it's a little too late. Watch for any other symptoms. Maybe some tetracycline in her water for a week.
I've personally found that trying to patch up sick chickens is like fixing a tractor by buying all new parts. Research symptoms on the web and you'll find from all the "experts" out there that you chicken's got everything from an upset stomach to the black plague.