I am not sure what model of mixer you have but if you have a 5qt bowl lift or a 4.5qt tilt head then it may be that your motor brushes have worn down or have been inserted incorrectly.
Below I have added a link to purchase new motor brushes:
https://secure.kaparts.co.uk/sku/6778011
I have also added a link below for our help video on "Replacing and removing the motor brushes"