Usually, a tree in this condition will develop a new leader from small branches just below the broken area. First cut the leader below the break and below other broken limbs that are no longer viable. If the diameter of the leader at the point of this cut is more than about a third of the base diameter, it is probably best to replace the tree. 

Next, determine the strongest and most upright limb at what is now the top of the tree.  This will develop into the new leader.  Pruning back other limbs around the new leader will encourage the new leader to take control.  Time will tell if this will develop into a strong leader and well shaped tree or if the damage was just too severe for the tree to recover from.