If they have removed all the bark all the way round the stem to any depth, then the tree will die from above that point. Sorry but all the living part of a tree trunk is in the cambium layer which is the pale brownish wet section of the tree just under the bark. It cannot be regenerated by the tree. Wait and see, but I fear the tree may be damaged beyond use.

hi Tosh,if they have nibbled a complete circle it is as owdboggy says the tree has lost its phloem and probably its cambial layer too. Bark wont regrow. If the tree was not good i'd use it as an excuse to dig it out and plant something 'more worthy' and remember to buy the plastic rabbit guard for it.