Did the top half start to leaf and then get damaged by a frosty wind? Failing that you are likely looking at a rooting problem where the root system cannot provide water/nutrients for the whole crown (top) so in an effort to stay alive the the tree 'balances' availability against need. If the top is dead, and there sometimes can be a second flush of leaves, then you may as well cut back to a growing point and hope the root/shoot ratio balances itself again. Be careful about using fertilisers at this point as a weakened root system may be damaged by them.