The gearbox in my Mitsubishi Magna has stopped working

It definitely could be the transmission’s main pump. Without this pump working properly, it can’t create the pressure necessary to select gears via the valve-body or indeed, produce enough drive force to actually get the car to move.

But, you could also be looking at a broken valve body, faulty selector cable from the cabin to the transmission, a broken torque converter or a complete, catastrophic internal failure of the gearbox. Instead of guessing, take it to a transmission specialist who is very likely to have seen this very problem many times before.