What do I do if the leaves fall off my plumeria?

You will need to pinpoint your problem first. Does your plant have bugs on it? Is the soil extremely wet, or extremely dry? What temperatures are you exposing your plant to? Is it in too much sun, or too little? Are you fertilizing too much, or too little? Any of these things can cause stress to the plant, and when a plant is stressed, it often responds by dropping leaves. That being said, your plant will lose its lower leaves on purpose as it grows larger leaves on top. So if the plant drops a few bottom leaves and then seems normal, it's fine. If it continues to drop leaves, and/or change leaf color, you need to examine your plant's environment closely to find out what is wrong.