Can you telnet to port 110 of your server? Do you get an error?
Do you have any errors in /usr/local/psa/var/log/maillog ?
Plesk defaults to a max of 40 connections and uses xinetd for managing the connections.
If you don't see any errors maybe something just got futzed up with Plesk.
You can restart pop with " /etc/init.d/courier-imap restart " (Yes, that has the pop in it too)