manage fields is missing
Did you flush all caches under Performance settings? You can do it manually too. Go to phpmyadmin and clear all tables with "cache" preposition. If you use admin_menu module, it need to be rebuild from time to time. 

Try to access manage_fields manually on path https://www.yourdomain.com/admin/structure/types/manage/page/fields


page is name of your content type


When I'm facing strange problem I always try to reproduce it on new Drupal installation. It tells me a lot when I'm "reverse-engineering" the problem.