To run Plesk, MySql service needs to keep running. Plesk Panel will become inaccessible if the MySQL service is stopped via the Plesk Panel & there will be no way to start the MySql service back from the Plesk Panel. This is why MySQL service should only be managed from the command line.
You can use these commands to restart the MySQL service:
1. service MySQL restart
2. service MariaDB restart
3. service mysqld restart