Check your database to see if the revisions are still there. Use phpmyadmin or another database tool and go to wp_posts table and search for post_type = revision. If they are not there, they are gone. A lot of “optimization” plugins remove post revisions for you. I would check if any of your plugins has that functionality.
If you still see then in the database, than you have another issue at hand, and will need further troubleshooting to figure out what’s going on.