It sounds like the power from the battery pack is no longer reaching the controller. maybe one of the wires is broken under its insulation, or there is a wire connection problem inside of the controller. Have you tried testing the battery pack Voltage when the battery pack is connected to the controller and the power switch is on? The reason I ask is that maybe one of the batteries has failed and the pack Voltage is dropping down to zero when there is a load on it.
Mike Pat