Hi

It shouldn't of turned brown may be one leaf or two at this time of the year BUT not the whole plant , what part of the UK are you , how long have you had it and the big question did you keep well watered all through the summer .

Gg

Have you had a frost? That could cause the leaves to go brown and fall off. They may come back next spring if the frost wasn't too severe but you would probably do well to keep it frost free for the rest of the winter.