Hi Susan and welcome to GoY. Sad to say but I think you have an ex-Magnolia. You don't say where you are in the UK but a lot of Magnolias and Camellias died due to the early winterly spell last November.

Prune off any dead wood - is it possible that its situation is different from the other one you have in that it receives more sun? If you're in the south or east, we had severe drought starting around the time this would have flowered - it was also extremely hot throughout April, so any flowers may well have been fried in the sun. Coupled with insufficient water supply, the plant has died back - if its in a more open situation than the other.

I also would suspect drought as the culprit. Magnolia x 'Susan' is a hardy deciduous hybrid, and shouldn't have been affected much by frost. The buds might have frozen if they were trying to open when the last frost hit, but new growth should have started soon after, if that was the only problem.