I'm sure we had this question very recently... Magnolia's don't like being moved and they don't like being in a pot so ideally you need to plant in the ground as soon as possible. In a similar conditions to those you had it growing in before. Keep well watered over the next several months until it has had a chance to settle and don't be surprised if it drops all its flower buds.

Oh my fingers are crossed for you !! I lost a magnolia in our last house when we had a conservatory built. Plenty of water, I would try anyway good luck.