First, try calibrating the ESC.



Check the sensor cable is plugged in firmly to the ESC and the motor.

Inspect the motor and esc tabs and make sure that there aren't wires or pieces of solder 'bridging' or shorting the terminals.

If you receiver has an antenna, inspect it for cuts or nicks. Try a different receiver if you have one.

Try rebinding the radio and receiver.

Good luck!