Possible improper transplanting technique, root disturbance, root ball allowed to dry out, delayed or inadequate watering after transplanting, fertilizer burn? Or is it a deciduous plant? 4 digit postal code, is that Australia? Are you into the fall/winter season when some plants drop their leaves as they go into dormancy? With no photo or information on the type of plant and the growing conditions, it's impossible for us to know what happened.
Bend some of the smaller branches and if they are still flexible the plant may still be alive and will re-foliate. Scrape off a thin layer of bark from a woody stem to determine if there is a green growing layer still viable. If so, keep up with appropriate watering and give it some more time. Don't apply chemical fertilizer until some new growth appears.