First, check to see if the key switch is "On". If the battery pack is charged and working properly, you will see the green "power" light on your thumb throttle illuminated.

Make sure the batteries in the battery pack are connected properly - to each other and to the end power supply/charger port wires. Plug the smart charger into the battery pack charger port. If it goes amber, it means that the batteries are connected and it the pack needs to be charged. If the light stays green: either your pack is okay and fully charged, or the battery cables inside the pack have become disconnected. Did you drop the battery pack recently?
Open up the battery pack and make sure that the batteries are connected properly in series. (See battery connection diagram.)

Probably not. The Hall Effects Sensor wires may be damaged or severed. Has the bike been dropped on the side of the hub motor where the cables come out? Did the hub motor ever spin inside the bike's front forks - twisting the cable into a ball or a knot? If so, the Hall Effects ribbon cable may be damaged. This will require you to return the hub motor to us for service.
But first, try this: make sure that all of the cables coming from the hub motor are connected properly to the cables going to the controller - red to red, blue to blue, etc. The old style controllers had three separate power wires that sometimes became unplugged. The newer wiring has a three-wire connector - eliminating the possible confusion on how to plug them in. If one of these wires is disconnected, your hub motor will vibrate or "chatter."

If you have checked the battery pack and determined that it is working properly, and checked all of the wires and determined that they are connected properly, you may have a failed controller.
Did you ever smell a burnt odor when you turned on the key switch?
If you have exhausted all of the other failure possibilities, the culprit is probably the controller. Please return it and we will replace it with a "Pedal First" controller, since we have no way of knowing if the Hall Effects Sensor wires have been damaged, which caused the controller to overheat and fail in the first place. If the Hall Effects wires coming from the hub motor are severed and you plug in another new controller (not a pedal first controller), this new controller will immediately burn up when power is applied to it.