There is a chance the wires going to the on/off switch may have been damaged, this is a common fault but usually shows up on older markers. Take the back plate off, be careful there is a spring underneath for the breech pin, and check the wires going to the switch. Give them a light pull if they come off there's your problem.
Also take the grip frame off and look at the battery wires, to see if they have been trapped anywhere and stripped. The battery could be getting shorted out by the body.
Also make sure the battery plug is inserted into the board fully.