Theft lock system

Drivers door was locked couldn’t get in so took door handle out and then it started beeping. So I took battery cables off and back on. Still beeping then went to start and nothing, no sound. Didn’t think about theft system bought used only thing was told was radio wouldn’t come on for some reason. I haven’t check fuses. Don’t know what exactly to do or where to look for anything or how to do the reset on this car.