Although frustrating, this is a fairly common problem. Plant roots and crowns survive, insulated from freezing temperatures by the soil around them. But the top of the plant dies, exposed to the cold wind and weather. Cut the dead tops off. Then if you want a single-trunk tree form again, choose the strongest sucker at the base to keep and prune off all other suckers. The stem you leave may need to be staked for a couple of years. Leave all of the leaves on it for the first year. It'll use every bit of energy it produces to grow. Next year start removing the lower leaves and side branches along the trunk. With this simple training and a few years' growth, you'll have patio trees again.