Improperly Connected Hall Sensor
Before buying any new parts, make sure that the connection between the control board and the hall sensor is nice and snug. The hall sensor, a grey wire with a three pronged end, is connected between the speed lever and control board. This connection could become loose due to the vibrations of the mixer. Also, the mixer will not work if the connection is not oriented properly. Using trial and error, change the orientation of the three pronged connection to see if the mixer will begin to work. The most common sign of a failing hall sensor is if your mixer increases to top speed and then shuts down.
Faulty Circuit Board
If your mixer is still not operating correctly, you could have a faulty circuit board in the back of your mixer. See our guide on how to replace your circuit board.