Plesk servers do not have a root mysql user, they have an admin user. This admin user is the same user used to log into the plesk portal.
Admin password should never be changed through mysql, it should be changed through the admin control panel.
To locate the admin password for plesk control panel, log into the server and run the following command:

/usr/local/psa/bin/admin --show-password

You can then log into plesk with the following address:

http://x.x.x.x:8443