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
