Usually, we see the mixers will stop spinning under a load. Such as a heavy dough, this is a sign of stripped gears. You can easily open this mixer up and check the gears with very basic tools. There are 4 main gears inside the mixer, and because they are all metal there is a good chance you have stripped more than one gear.
I would start by taking each gear out and cleaning it with WD40 and paper towels (unless you have access to a parts washer) then inspect each gear. Most of the time it's very obvious which gear is stripped as it will be missing several teeth. But any gear showing wear should be replaced.
You should also clean out the entire gearbox and pack it with new grease as little bits of the old gears can be in the grease and cause problems in the future.
You can find the full-service manual online at KitchenAidExperts.com or through this link: KitchenAid 6 Quart Service Manual (kitchenaidexperts.com)
You can also find all the parts you will need to fix your mixer on our website. www.kitchenaidexperts.com
Good luck on your repair and reach out if you have any questions.
Colleen @ KitchenAid Experts