When you plug your beaters make sure you can hear a “click” noise that assures they are plugged in all the way. If there is no noise when you stick the beaters all the way, the tabs could be broken. Examine the beaters for damage to the tabs, replace the beaters if necessary. If everything is good with the beaters, the mixer’s spring that holds the beaters could be damaged and it will need to be replaced. Check out the section on beaters falling out on the KitchenAid KHM512ER Troubleshooting Page .
If you were working with the mixer and it started making an unusual loud noise, it’s likely a food particle or other foreign object got stuck in it. You can disassemble the mixer, search the foreign object and remove it. If you cannot locate the foreign object, it is possible the mixer needs to be lubricated. Check out the section on noise on the mixer's troubleshooting page.
If your mixer won’t turn on make sure it is plugged in a regular electrical outlet. Check if the outlet is working using other device that you are confident will work. If the outlet is working, unplug the power cord and examine it for damages - damaged power cord will cause the mixer to not turn on. Also, check out the KitchenAid KHM512ER Troubleshooting Page for more details.