I'd check the fuse first. If it checks out as ok, then pop the motor out of the grip, reconnect its wires, and see if it turns. Even if the battery is shoddy and can't turn the motor over when it's in the gun, it should turn over fine with the motor out of the gun. If it works, then you're probably looking a locked up mechbox or the battery is screwed and too weak to turn the mechbox. If it doesn't turn out of the gun, then the problem is more than likely the wiring.
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