Since you have had this tree for several years now I will assume that you have a handle on watering and fertilization. Have you by any chance changed it's position or had any unusually cool weather?
When was the last time that you have re-potted it. It is possible that the soil has collapsed/deteriorated to the point that drainage is impeded. When you water does the water drain quickly or puddle on the top of the soil?
At any rate don't panic it may recover. Make sure that you are not over-watering it. Whatever your routine was before its water requirements will be lessened if the plant is not doing well.