You may have already put a plastic cone around it but it is not covering the entire trunk.  You need to either get another one and cover the exposed area or wrap your tree trunk with hardware cloth (found in big box stores near the chicken wire).  Leaving part of the trunk open exposes that to the rabbits and they will nudge the open sides open even more.  There is nothing you can do to coat the trunk but must allow it to seal over on it's own.  If the bark is not remove all the way around the tree, it should be okay.  If it was completely girdled, then the future is dim.