Chinchillas can break bones during fights, getting a limb stuck in wire mesh, from a bad fall, etc. Sometimes the only signs will be that the chinchilla has difficulty moving or favors certain limbs. With a bad break bones may even protrude through the skin. There is not much that you can do. Stop any bleeding by applying pressure to it with a clean bandage or cotton swab. A sanitary pad works well. The chinchilla will remove it if left alone so take it to the vet ASAP. If it happens during the night and a vet is not available, put the chinchilla in a small holding cage to limit its movement to the minimum. We have a couple of wooden boxes designed for transporting birds that we keep especially for this purpose.