Transmission Problems? Ask a Mechanic for Answers ASAP
If the transmission feels like it slipped to neutral there are very few possibilities here that are outside of it being an internal failure of the transmission. You did not mention what year or model vehicle it is, however all Mitsubishi transmissions have a fail safe built in to provide 3rd and Reverse in the event of an electronic failure. If you unplug the transmission electronics and you still have no movement at all like it is in neutral, then the transmission has an internal failure (either clutch wear or hydraulic pressure loss). You would need to get it to a transmission shop for a line pressure test to see if hydraulic pressures are normal to determine if you were only looking at a clutch issue or if you have a more severe internal failure.
In almost all scenarios however, depending on the vehicle model/year to some extent of course, it is more cost effective to replace with a known good unit than it is to overhaul the transmissions due to the cost of parts for Mitsubishi transmissions.