bringing in potted bougainvillea

Question

When I brought my plant inside for winter, all the leaves have dropped off.

Answer 1

You should bring the plant indoors when the temperatures are about 42 degrees F during the daytime hours.
A south facing window is best to give it plenty of light.
This light does not meet this sun lovers needs, but it should keep it growing through the winter.
Let the soil dry out between waterings.
You can also keep the plant in a cool location; between 32 and 44F. This will force your plant into dormancy. All the leaves will drop from your plant.
Water just enough to keep the soil from drying completely-once or twice a month.

https://www.gardeningknowhow.com/ornamental/vines/bougainvillea/bougainvillea-in-containers.htm