|Our pepper tree has suddenly lost a great deal (most) of its leaves|
|You didn't say how long your tree has been in the ground. If it has been there a number of years, I'd suspect a watering problem. If it has recently been transplanted, it may be going through some transplant shock.
Once the leaves are lost, there is very little, if any, water movement up the tree because the leaves "pull" the water through its conducting vessels along with some help from pumping pressure from the roots. Therefore, it is important that you do not over-water your pepper tree while it is in this condition. Since water will not be pulled up by the leaves, the branches will begin to dieback until it sends out new leaves.
How rapidly new leaves develop will determine the extent of dieback for the most part. If there are no buds for the tree to push new growth from (this is the time of year just after growth in the spring and the development of new buds for the winter) there will most likely be major dieback if the tree recovers at all.
I'd give the tree a chance to recover. If it doesn't leaf out by next spring, you may have to replace it.