You are able to control your servers services using the Plesk control panel, however you can also control them via SSH also.

It is possible that MySQL or Plesk stop and the only way of restarting the services is via the command line.

Redhat based (fedora/centos)

MySQL:-

/etc/init.d/mysqld stop

/etc/init.d/mysqld start

Apache:-

/etc/init.d/httpd stop

/etc/init.d/httpd start

Qmail server:-

/etc/init.d/qmail stop

/etc/init.d/qmail start

Plesk:-

/etc/init.d/psa stop

/etc/init.d/psa Start

Debian based (Debian/ubuntu)

MySQL:-

/etc/init.d/mysql stop

/etc/init.d/mysql start

Apache:-

/etc/init.d/apache2 stop

/etc/init.d/apache2 start

Qmail server:-

/etc/init.d/qmail stop

/etc/init.d/qmail start

Plesk:-

/etc/init.d/psa stop

/etc/init.d/psa start