Arduino complete self shutdown
Firstly as Tut pointed out with the current schematic the body diode of the FET is conducting so it doesn't matter what you do it'll always conduct. And you are using a p channel on the 0V line, that means you'll need a negative voltage on the gate to control it.

Either switch Q1 to an N channel part with the source connected to your battery negative terminal or change the circuit so you are controlling the positive supply rail with the FET.