MySQL Daemon failed to start on CentOS 6 after automatic update
That issue is related to a broken update by Plesk.

Please read the article https://support.plesk.com/hc/en-us/articles/115001906725 .

You can fix it by running this command:

install -m 0700 -o mysql -g mysql -d /var/lib/mysql-files


and start mysql:

service mysqld start