Fix Plesk 8.1 Server

I have a dedicated server running Plesk 8.1. The server had been up for several months with no issues. Yesterday, I attempted to install Zend. Since that time, I can not resolve any of my sites. The control panel is accessible via a browser and so is the server via SSH. I need someone very familar with Plesk and Apache to determine what the problem is and get the domains back up and running. Further, before becoming unoperational, the server was not processing php files correctly. I would alos want this to be looked at. Thanks