Chameleons in the wild use a "suicide drop" to avoid being eaten - It's quite scary if you've never seen it - there are other reason also that can cause them to fall - click on Resources and check the caresheet for your chameleon (if you got a care sheet from the pet store throw it away and make a copy of it for reference instead)
You didn't say what kind of chameleon you have I am assuming she's pretty young ? If she's not a Jackson or a live bearing species there is information on laying bins - and when you need to have one.
Most likely something startled her and she did the drop- and it's nothing to worry about - or picked a bad branch- It's always a good idea to check the care sheet though -