1) Plesk Control Panel shows the error: Service Unavailable - HTTP Error 503.
2) In event viewer, system logs show:
Application pool PleskControlPanel has been disabled. Windows Process Activation Service (WAS) did not create a worker process to serve the application pool because the application pool identity is invalid.
Password for psaadm user in IIS->Application pools->PleskControlPanel pool does not correspond actual password for psaadm user in Windows users.
Launch Plesk Reconfigurator->Repair Plesk Installation->Plesk Services.