2003 mustang all power works but will not start

2003 mustang all power works but will not start 2003 ford mustang ran fine and started fine. when installed new radio it started fine and then stopped starting. Checked all electric and it was fine. removed new radio and still will not turn over. Had battery checked and wires and this has a thief lock system on it and tried to dis connect batter and still will not turn over and all the other system turns on. Any help?