Looks like it got broken, hard to say how. Could have been caught in wire, sometimes that happens when they try to get something on the other side. I would just leave it alone, it should grow back out with time. Only time I would mess with one is if a really sharp edge needed to be filed off to prevent catching it on something, or if a crack was progressing and getting worse. It might be sore for a few days, so make sure she's eating and drinking well. If she needs help then making a mash of their feed by mixing in water and putting in a deeper dish raised up can make it easier for them to eat. She might have a bit of trouble picking things up off the ground at first.