speed reading drops to zero: there are two possible reasons. 1) your bike does not have an external speed sensor, the controller counts the motor pulses and the motor has stopped, we still don't know what causes the motor to stop.. 2) your bike has an external speed sensor (you see a magnet bolted to one of thee spokes that passes in front of the sensor) the controller is faulty.
You need to borrow a controller from Kudos to test your bike.