Have you checked the position of the "M/A" lever to see if it's stuck in an intermediate position? It could be preventing the autofocus from working. I often shoot with a long focal length, and I've had a similar problem with a zoom lens where the autofocus motor would get stuck. Simply wiping it down with alcohol didn't solve the problem. I was thinking about taking it apart, but first checked the website https://manualmachine.com , to find a manual. And it helped me. A PDF had a full description of how the lens works. I managed to fix the problem using the manual. There's also detailed information on how to properly calibrate the front-back focus via the service menu, and it also helped me a lot.
