Q.leaves fell off bougainvillea
why did all the leaves and flowers fall of my bougainvillea and what can i do about the problem

Flower and leaf drop could be due to stress, pests, disease or watering issues.
Look for signs of issues on the leaves and plant. Do you see pest or holes in leaves?
Is the soil wet and soggy or dry?
Is the soil well draining?
Is the plant in the correct location?
This article will help refresh you on the care requirements.
https://www.gardeningknowhow.com/ornamental/vines/bougainvillea/bougainvillea-flower-drop.htm