This happens with my Radcity3 ST. I power it down and back up. I will start getting power to the motor after doing this 2 or 3 times. I took this in to the Radpower bike shop for service and had them look at it. They said they couldn't find anything wrong and suggested the power off-power on solution. On one occasion, it was a loose connection from the handle bar controls, so do check your wire harness connections.

I also occasionally will not be able to get my lights to turn on, but get power to the motor. I mounted a secondary headlight for those times when I can't get the lights on. It usually resolves itself by the next time I ride.