That's not such an unusual occurrence. The Plesk control panel is a bolted-on set of scripts which you interact with through the web server process. The fact that Plesk is unreachable means that there is likely something wrong with whatever http server you are running or with the Plesk scripts themselves (if they've been updated, perhaps).
Here are some tips:
- Check the Plesk error logs:
- Check your HTTP server's error logs.
- CentOS Apache error logs:
- Reboot your HTTP server
- "Reboot" Plesk
~# /etc/init.d/psa stop
~# /etc/init.d/psa start
- Reboot your VPS (although, that's not a solution by any means)
That should get you well on your way.
Since it appears that the psa file does not exist, you appear to have a serious problem. Check your bash history to see what commands have been run. Perhaps something was done to uninstall Plesk or delete critical files. I hope you have a backup. You may need to re-install Plesk. You now know what specificall is wrong with your installation and can either make another question here on ServerFault that specifically addresses the problem (E.g. "Help, /etc/init.d/psa is missing") or go to the Parallels support forums and ask for help. If you are using a host that has installed Plesk for you then they will be able to help. If you have purchased it yourself then Parallels will be able to help.