I just did the same thing, i.e., had a "spark flash" on 2 different KitchenAid stand mixers when replacing a brush (stupid of me to have it plugged in and power on while performing this operation) and thereafter neither would run again. There is a small circuit board on the rear end of the mixer, in my case Whirlpool P/N WPW10325124 BOARD-PHAS. I purchased 2 new phaser boards, installed them and both mixers worked perfectly thereafter. The link below provides instructions on how to replace the circuit board.
https://www.youtube.com/watch?v=JkBLtuHPiLY