A website hosted on Plesk server was showing below error:

HTTP Error 503. The service is unavailable.

When checked the logs available in Plesk control panel at Domains > Domain.com > Logs, there are no errors found.

The real cause of the error was ‘IIS application pool is stopped for the website.’

The below error message was showing under Domains > Domain.com > Dedicated IIS Application Pool for Website:

Error: Your server's applications pool is not running, but it was not stopped through Plesk.
There might be some issue with IIS. You can restart the application pool.

We can solve the issue by Restarting the website application pool.

To Restart, Go to Domains > domainname.com > Dedicated IIS Application Pool for Website.

Click Switch Off

Click Switch On.