Your server's applications pool is not running, but it was not stopped through Plesk.
Cause
The website's IIS application pool is stopped.
Resolution
Start the website's application pool:
Via Plesk
Log in to Plesk.
Go to Domains > example.com > Dedicated IIS Application Pool for Website.
Click Switch Off.
Click Switch On.
Via RDP connection
Connect to a Plesk server via RDP.
Start a command prompt as an Administrator and run the command:
plesk repair web example.com -y