Hi. My name isXXXXX a camera repair technician with over 20 years in the business and I'll be glad to help you with your question today.
If you have other lenses, try them on the camera first. Replacing the AF motor on the 100-400 gets expensive. You might want to get a few estimates from independent repair shops and compare them to a factory repair. Below is a link to a directory of repair shops, one of which should be in your area and able to help you.