If it won't come out when you tug on it, then absolutely don't do that again. There are some links here that have great illustrations about force feeding chickens. The point I'm thinking of here is the pics there might help you see if the string is wrapped around the chick's tongue. Hold on a sec let me see if I can find the link.
OK, I had more luck searching google images for chicken tongue.
So, go ahead google that and have a look at how their tongues and mouth work. The string may be wrapped around the tongue. If you have a helper, a strong flashlight and some tweezers, see if the string is wrapped around the tongue, and try to gently work it off if you can.