First thing you need to do is make sure that it was her that took it. Then what I would do is confront her about it. I would sit her down and talk to her about the facts. While trying not to get too flustered, you need to try and tell her that it is your property. If she does not agree to give it back you can do nothing about it because I am guessing that you are living in her house. It would be a shame to not get it back, but next time you just have to be a little more careful.