After upgrading MAMP Pro from 3.4 to 3.5 my MySQL server wouldn't start up. All possible solutions that I could find on the net just were not working for me. Then it came to me. A while back I upgraded my MySQL server from 5.5 to 5.6 within MAMP and that update gets lost when you upgrade MAMP itself. So I re-ran the MySQL upgrade script and boom, my MySQL server started up!