This is the plant?
Lady of the Night (Brunfelsia 'Gigantea')
Maybe it got too much sun. One site said "Part Shade to Shade", but another site said it could take some sun.
Maybe it got too cold - it seems to be a heat-loving shrub.
Did its growing location just get brighter or colder as the sun moved with the season?
Where do you live, or rather, what is your local climate like?
Is it in a container? Is it root-bound?
I don't know that particular plant, but I would guess that if almost all of it turned brown (like, dead-brown, necrotic), it's too late for anything except maybe trying to take a cutting, or see if it will come back from roots. On the other hand, it is a shrub, so I don't know if it can die back to the wood and still return. Perhaps next spring.
If just a few leaves turned brown, trim them off and carry on after correcting whatever condition caused the problem.
Can you take a photo and post it? And provide more details?
@wcgypsy contributed a photo, so maybe she has grown it.
