If you get the message “Service Unavailable” while accessing Plesk:

Check the EventViewer – Open Administrative Tools >> Event Viewer.

There if you see the following W3SVC-WP (IIS6) error messages:

Could not load all ISAPI filters for site/service. Therefore startup aborted.
OR
The HTTP Filter DLL C:\Program Files\Helicon\ISAPI_Rewrite3\ISAPI_Rewrite.dll failed to load. The data is the error.

To fix this, all you need to do is -

1. Remove filter from IIS manager:

3. Restart IIS

Happy Bookmarking!