Call the police? Form a posse, put in some Beyonce (or Wagner), and go kick the offender in the balls?
Why are you asking this? Did you lose yours? If so, you might want to give us details so we can help.
Otherwise, what can we say? There are apps and so forth that are supposed to help. You can look in the app store for those if you want. But, if you already lost it, you are out of luck.