I had a magnolia tree for about 15 years

Did you have a day or night of close to frost or freezing temperatures? If so, flowering for this year will be disappointing but don't worry your tree is just fine.

So long as its in the ground and not in a pot, and the plant is otherwise fully healthy, with healthy leaves and stems, likely, as Loosestrife says, its variation in temperature or cold temperatures, especially if its flowered earlier than usual because of the mild winter.

Our Stellata cross is just showing its first flowers, I dread a frost as its just turns the edges of the petals brown.....like the camellia next to it, more sunny mild days and nights needed.......
They do face west .....

You must of had a frost or two, probably overnight. One good frost usually does it, Magnolias in pots are particularly vulnerable.