I recently had the need to restart Plesk on a server. Everything on the server was running just fine, but Plesk was not responding.

After some searching I found these handy lines that allowed me to get Plesk back up and running.

# /etc/rc.d/init.d/psa stopall
# /etc/rc.d/init.d/psa start

Note: Must be run as root, or su.