What were the warnings? Did the upgrade script say OK at the end or did it finish with an error?
I experienced this in the upgrade from 2.4.8.2 to 3.1. It turned out that I needed to install a clean copy of OJS from a tarball downloaded from https://pkp.sfu.ca/ojs/ojs_download/, had to drop the database and install it again from a backup (I had to fix some problems with duplicate entries first and problems with encoding of tables, they need to be utf8). I also restored the /files directory from a backup and gave it permissions 777 temporarily for the upgrade process, and added settings for the database login and files directory in the config.inc.php. Then I ran the upgrade script from the shell using php tools/upgrade.php upgrade. The upgrade process is long, as it is moving files around. You should check if it ends ok or it gives you any errors.
Below you can find my experience (I started first with an installation from git, but then switched to the full package from the ojs download site to simplify my life):