My wifw was using her mixer, turned off to add an ingredient and it wouldn't turn back on.

It is most likely that the 'shear pin' in the drive system has sheared. If it was 'new' someone bought it, over-worked it, packed it up and sent it back.

https://www.google.com/search?q=Sunbeam+2370+stops+spinning

Parts are obsolete already, it must be new to you, not 'new'.

https://www.ereplacementparts.com/sunbeam-2370-stand-mixer-parts-c-116440_116448_116560.html

It you enjoy baking get a KitchenAid stand mixer.

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Some ...what you may consider silly stuff...
1 check the outlet that you are plugging your mixer into it may be bad. Not the wire but the plug in the wall may be bad, plug a hair dryer in on high if it runs for 60 seconds it ok.
2 check the plug of the mixer make sure it is not bent at the wall pins
3 open the rear of the mixer by removing the round cover at the rear of the mixer look for loose wires, sometimes the socket connectors on the speed switch break off.
4 there are three screws on the speed control switch that control the mixer, sometimes after time these screws can actually turn due to vibration and the mixer will just not start....so.....start with the top right screw and turn it counter clock wise a quarter turn and start the mixer if it starts even for a half a second then the problem is in the switch. and your only problem is the proper timing set by the three screws.
5 gears inside the mixer can strip and freeze the motor so nothing turns...put your ear next to the machine mid way and turn it on ...if you hear a hum from inside then it is a gear problem.
6 If your machine is older then 1978 then it may be the IR filter inside the mixer (hard to find )
7 the carbon brushes that are located on the side of the mixer may have finally worn down (cheap fix) but there are four ways to put the new brushes into the machine and only one way works. You have to be careful with the beveled edge of the new brush and which way it fits back into the housing.
8 Now you are inside the mixer if everything else fails, the motor is the next step and it will require a full tear down, to repair, there are two parts the armature and the winding and each will take about thirty-five bucks to get and either or both will take about 2 hours to repair but if you go all the way down then you need a new gasket and gear grease clean and re-pack too. At this point you are performing an overhaul . let us know if this helps.

sorry to say but you have an inside gear problem, not catastrophic in and of itself, but it will require a full tear-down, the worm gear is either worn down or stripped, It is made of nylon and its supposed to wear out or give out if there is a problem. It appears that there is enough gear left to turn the paddle but as soon as you have any load on it well not enough tread left to keep spinning the paddle. Service center will cost from $150.00 to 200.00 depending on where your nearest KA service center is (that includes shipping) If you are good with mechanical stuff you can do it yourself for about $50.00 dollars. There are sites on the web that will show you how and your parts list from the kitchen Aid site will show you what you need to order off of ebay to repair it.

What model number is your machine? Try turning it on it's side and shake a little, take a very small screwdriver and grind away to crack the brush and shake it out. Otherwise it is a full tear down to take the brush holder out. Hope this helps if not get back to us for help

Did you play with the Brushes??? If you did you have to make sure that they went back in the correct way or they won't be touching the armature and the mixer will not operate untill the brushes touch the armature. If you did nit take the brushes out at all. then your problem is the flag connectors are not in the correct terminals.
the other problem is the possibility that when you re-installed the speed board that you now have to tune the mixer by adjusting the screws on the top of the speed board.

Hello,

Note, electric mixers are great to have around, whether the stand or hand-held type. For those of us that use one on practically a daily basis, here are few instruction below on how to fix an electric mixer.

Instructions

1. Unplug the mixer before attempting any type of repair. If the mixer has a lot of batter and food on it, clean the appliance. A lot of times while using the mixer, the ingredients can work their way up the beaters and make their way into the gear section where the beaters attach. This can happen especially when mixing bread dough, cookie batter or any dense or heavy ingredients.
   
2. Refer to owner's manuals when fixing an electric mixer. If the mixer stops beating when it's turned on, it could be that the appliance isn't getting the proper voltage. Test the mixer using a voltmeter. If the mixer is getting the proper voltage, it could be that one of the gears are worn out.
   
3. Check the wiring on the mixer. Look for any cracks, tears or exposed wires. Sometimes just using electrical tape to repair the cord is all that's needed.
   
4. Take out the screws from underneath the head of the mixer using a Phillips screwdriver. Next take out the screws on the back of the head of the mixer (some brands have these, others don't).
   
5. Grasp the head of the mixer and gently pry off the housing with a large screwdriver. A slot screwdriver is preferable. Clean out the area with a soft cloth and look for any broken parts or wires.
   
6. Go directly to the website of the brand of mixer you have. Most major brands will offer advice and suggestions for proper maintenance.
   

Good luck and thanks for using fixya.

It seems that the mixer is overheating and tripping the thermostat thus preventing further use. I would recommend a thorough overhaul. You will need to carefully inspect all the components, particularly Items the brush assemblies, and the commutator portion of the armature (where the brushes touch the armature), and everywhere else in the motor compartment. Any parts exposed to oil will need to be degreased and dried before reassembly. When you reassemble your mixer, the brushes can only be installed one way. Make sure the brush's concave contact face is resting completely on the armature. If it is turned it 90 degrees too much, it wont' seat properly and the motor will not get any juice.
While your mixer's apart you should check for any other unseemly wear and tear on the gears, etc. You may find more things that need fixing, adjustment or replacement.You may find it easier to replace the brushes than degreasing them. I recommend using a food-grade grease that you can order online (although I've seen automotive bearing grease used too - your call). If you feel this is beyond your capabilities and will be too difficult to do by yourself you can take your mixer to a local small appliance repair shop and have them troubleshoot and fix your mixer.

Hi COVINIO. Yes, the "large black spring loaded magnetic screws on the sides of the mixer" are your mixer's brushes, which supplies electricity to the electric motor. (The black cap is the brush cover). I suspect that one or both of the brushes is (are) not seated properly. Did you remove the brushes to replace them or were you just inspecting them, when you replaced your plastic worm gear?
The brushes need to be installed so that they both seat correctly on the armature's commutator. See the illustration below:

I'd suggest removing them again and making sure the concave contact face rests completely on the armature’s commutator. If you've turned it 90 degrees too much, it won’t seat properly and you get no power to the motor.
It would also be helpful to have the Parts List manual on hand. If you're lacking one, you can download a PDF version from the KA website at: http://www.kitchenaid.com/flash.cmd?/#/page/home.
Click on Customer Care>Locate Manuals & Guides> Enter the 6 character model number, in your case use KSM50P (Your K5SS mixer falls within this manual), and select the first file KSM50P.pdf (438.31 KB) listed under Parts List (4) and then check to verify this matches your mixer model.
Turn to pages 6 & 7, titled “Motor And Control Parts”, which provides an exploded view for the disassembly and reassembly of your mixer’s electrical components, along with all the parts. Locate Illus No 47, Brush holder assembly that includes #28, 32, 33, 34 & 46 which is the brush holder insert, and Illus. No. 29, brush & Spring assembly respectively. The illustration along with the picture above regarding the orientation of the brushes should get your mixer powered back up and operational. Good luck! I hope this helps. Howard, Burke, VA
If this solution has been helpful to you, please rate it, thanks! -hslincoln

I was able to take the mixer apart and found the gear that was worn out.

the brushes are not square, but have one corner cut off.  use a flashlight to see which direction the brushes must be put back.  clean the ends of them well with a cotton cloth before reinstallation.
try spinning the rotor-head by hand, then try turning it on (sometime junk is stuck between the brushes and the commutator (motor).