There may be two reasons for Mantis motor issues:
1. controller failure (likely)
2. motor failure (unlikely).
The best way to identify the problem would be to check if the wheel has resistance when turning by hand. In order to know for sure, you would need to open the deck and unplug the power from the battery. If the wheel does not resist anymore, it is certainly the controller.
Have a look at this guide on how to open the deck and some explanation of the cables inside.
How do I open the Mantis deck and what are the connections? If the wheel still has resistance, swap the motors - connect the rear motor to the front controller and front motor to rear controller - power the scooter and push the throttle. If the same defective wheel still doesn't work is a motor issue. If the wheel that has not worked is now working, is a controller issue.
Please also check if the cables are properly plugged into the motors.
Please let us know the results