Mixer makes terrible sounds then stopped working

Hello Emily, the problem is the internal gears have broken, stripped or popped loose due to heavy loads during use. The solution is to go to the KA site and get a parts list for you model machine first thing. You can download it from here
http://www.kitchenaid.com/webapp/wcs/stores/servlet/OCManualsLiteratureSearchResultView?catalogId=10552&langId=-1&storeId=10202
you will need the full model number for the correct parts list. then you will need about 30 minutes and an open work space and a couple plastic bags, a roll or paper towels and a little patience.
Yow ill be opening the machine cover removing it and placing the screws in one of the bags and mark on the bag with sharpie pen "cover screws", next you will need to remove the transmission cover (messy) and removing the gasket, toss the gasket (you will be ordering a new one $6.00 bucks) place the cover and screws in bag and mark. then look at the gears inside you will see several, rotate by hand I believe that you will find the culprit, it is more than likely the "worm follower gear" BTW if you open the machine and the transmission cover is black plastic check it for cracks, it was a major cause of this problem some years ago and KA has since relsoved the problem by the use of an all metal transmission cover, if it is cracked you will need a new one ($26.00 plus shipping) now find the broken gear and order a new one and replace it. Next if the grease inside your mixer is not all watery you can reuse it by scraping out with a popsicle stick and put in something that you can easily get it back out of for later use. if the grease is all watery you will need new ($7.00 bucks) (all the parts can be bought at several sites on the internet at once to cut down on shipping) Then re-assemble. There are several good if long winded videos on youtube that go through the tear down and replacement and re-assembly of the 600 machine that will guide you through process. Good luck and be patient.