A lot of tropicals go very dormant in the winter, dropping most, or even all of their leaves. I find it a little scary, to be honest, because I’m not at all sure that the plant (a bougainvillea, in this case) is still alive. You can bend the branches to see if they’re still supple — if they’re snapping, that’s a bad sign. But mostly, I end up just waiting and hoping, that when the warm weather returns and it’s save to return them to my sunny porch, the leaves and flowers will come back too.
In the meantime, some grocery story primroses underplanted add a nice note of cheery color in January, and don’t make me feel quite so despairing when I glance in this plant’s direction. They should last a few (or even several) weeks, and when they’re finishing up, you can dig them up and plant them in the garden, in a nice part-shade woodlandy spot, where they should be perennial (in zones 4-8), returning for a number of years and even spreading by seed