Kitchenaid classic motor runs, "whirls" (you can hear the motor spin) but beater won't turn under any pressure, will turn with water/liquid but nothing with drag like dough. Had been mixing fondant.

You may need to adjust the head height if the new flat paddle is a bit bigger than the original. When you have the head lifted up with beater out of the bowl you will see a single slotted screw where the head hinges on the pedestal. Anti clockwise to lift the head up, clockwise to drop the head lower. Paddle should just clear the bowl with head down & motor running.

a gear or gears. their usually plastic and will damage before the drive shaft

the lower setting is for stirring. this is a high quality machine that is made to take a beating. you are just hearing the motor wind up. think of it like a jet engine starting, its made to run its smoothest at high rpm's, so it sounds funny at low rpm's but nothing is wrong with it.

Hi, Did you push the beater in so that it clicks. If all is good there, gear box maybe at fault or jammed. Take it to a service center for repair. If you are machanicly inclined (DIY) Take it apart and see why its not turning. Good luck.

Hi ERIN. This should be pretty easy to do by yourself. The instructions for adjusting your beater to bowl clearance is in your Owners Manual. If you don’t have the original manual you can easily download a PDF version of your Owners Manual from the KA website: (http://www.kitchenaid.com/flash.cmd?/#/page/home).
Click on Customer Care>Locate Manuals& Guides> and enter the 6 character model number, in your case K$45SSW and select the file listed for your model that appears under Owners Manuals (2): K45SSWH-0.pdf (2730.37 KB) and save that file to your PC. Turn to Page 13 and you will find the instructions for adjusting the mixer head height. I have extracted the instructions below:
Beater to Bowl Clearance
Your mixer is adjusted at the factory so that the flat beater just clears the bottom of the bowl. If for any reason, the flat beater hits the bottom of the bowl or is too far away from the bowl, clearance
Tilt-Head models
• Unplug mixer.
• Lift motor head.

• Turn screw (A) SLIGHTLY counter clockwise (left) to raise flat beater or clockwise (right) to lower flat beater.
• Make adjustment with flat beater, so it just clears surface of bowl.
If you over adjust the screw, the bowl lock lever may not lock into place.
I hope this helps! – Howard, Burke, VA
If this solution has helped you, please rate it, thanks! - hslincoln

This sound like the worm gear has failed: http://kitchenaidmixerspares.co.uk/Home/tabid/36/p/Mixer-Part---Worm-Gear-and-Bracket-240309-2/ProductID/103/CategoryID/17/Default.aspx
You will need to take the mixer apart to confirm this.

This happened to me also, and it was because one of the beaters was no longer straight (bent just slightly, but enough). I found a replacement beater at a thrift store, and now it is fine. There were several replacement beaters at the thrift store... I'm guessing because many of the KA hand mixers are starting to die (the beaters don't 'click' in anymore).

It sounds like it has been overloaded and the worm gear has been stripped. You will need to take the head apart to confirm this.

take off the chrome ring around the rotator. remove the screws that were hiding under it. pull off the rotator and replace the "O" ring on the shaft.

http://www.fixya.com/support/t636606-kitchenaid_ksm90_stand_mixer

Kitchenaid service manuel gives step-by-step instruction on a stand mixer.