Mysql has a authentication record for different hostnames and IP addresses, so root@fqdn can have a different password then root@localhost. Seems as if seomone changed the record for the fqdn and not for localhost. Or someone changed the /etc/hosts file so that there is no valid localhost entry in there anymore.
Check the /etc/hosts file and make sure that localhost is mapped to IP 127.0.0.1. If thats ok, login to your mysql db with phpmyadmin or on the shell and set the correct root password for localhost.