I was not aware of an anti theft feature on Android.
You can lock it from your google account if stolen or missing, then unlock it easily when you find it again by entering in your email.
Have you changed the sim card?
One of the things that will trigger the software is if you change the sim card.
Put it back in, and see what happens.
Or just enter your password... but i am guessing you have forgotten it, or don't know it because you are the thief