Magnolia flower buds stick out on the tips of the twigs, so they are very subject to frost damage in a hard winter like the last one. Other possible problems are heavy pruning too late in the summer for the tree to form new buds, too heavy feeding in the early spring, or chemical damage from weed and feed in late summer. If this has been a developing problem, the tree may have been gradually getting too much shade from a fast growing shade tree nearby--they like some shade, but if it gets too dark, they will stop blooming.