Sorry to hear that, I wonder what could have happened? Maybe you can put some antibiotic ointment on it or something for the time being. If the other chicks are pecking it you should probably separate it until it heals.
Are you in California? If you post on the Northern California board people are really knowledgeable & helpful there.
Here are some of my chicks with the broodies:
I have 3 black chicks, 2 lavender, 1 chocolate, 1 blue and 1 lemon cuckoo, waiting on 3 more eggs to see if they hatch.
Good luck with your chocolate chick; I hope she is better soon.