KitchenAid 350 Watts KSM50PR Professional Series Stand Mixer stuck on

I would recommend that you first check the Hall effect sensor which determines the speed.  I had this exact problem and I actually went out and bought another Hall sensor for about $13.  I replaced the original sensor and made sure that the connection between the sensor and control board was nice and snug. My blender worked as good as new! But I was curious to know if my original sensor was defective so I put it back in mixer and again made sure all of the connections were tight.  Guess what?  My original sensor also made my mixer work!  There was nothing wrong with my original Hall effect sensor except the connection between the control board and sensor was not adequate--unfortunately I didn't check the connections before I decided to buy another one. My guess is that the connection between the control board and the sensor probably loosens or comes undone over time from use and vibrations. 
1.  Download the mixer parts and diagrams from the Kitchenaid website to see what the Hall effect sensor looks like. 2.  You'll need a phillips head screwdriver to open up the mixer. 3.  Make sure that the connection between the control board and sensor is absolutely tight and snug. 4.  Also, make sure that the orientation of the sensor connection between the control board and sensor is correct. it's hard to know which way the sensor should be connected because the control board has three prongs and the sensor can attach facing up or facing down. If, after you've made the connections, the mixer still doesn't work.  Disconnect the sensor and reconnect it with the prongs facing the opposite direction and try it.  I actually found the right orientation through trial and error.  By going through this trial and error, I also had my Eureka moment when I realized that the original sensor probably would also work if it were reconnected with correct orientation.   5.  This whole process probably takes less than 10-15 minutes to perform.  6.  Make sure you use common sense and unplug the mixer when making connections and such.