Just leave it alone for now. That was a big hit that plants in your area took and it could take time for them to recover. Fertilizing is not advised as it only adds stress to an already stressed plant and they cannot process and absorb it. And will do nothing to hasten recovery anyway.

If the tree is sprouting from the base only, it most likely froze to the ground. The top part is probably dead. You can select a new trunk from the sprouts at the base and grow a new tree. The trees roots and root flare at the base of the trunk are supposed to be at the surface, not buried a foot below ground.

Test the old part by snipping a branch. If it's green inside, it's alive. If brown and rigid, it's dead. Prune it away and concentrate on the new shoots.