Fix HTTP Error 404.3 - Not Found at index.php
HTTP Error 404.3 - Not Found
The page you are requesting cannot be served because of the extension configuration. If the page is a script, add a handler. If the file should be downloaded, add a MIME map.
There is very likely no PHP handler in place for this site in IIS. Thus, there is nothing available to serve the file properly.
You will need to go into Plesk, search for the affected domain, and navigate to the domain's control panel. From here you can click the domain to manage its WebHosting Settings. On this page, you can manage things like ASP, CGI, Perl, and of course PHP. You should just be able to disable and re-enable PHP to re-add the handlers.
If you run into any errors about duplicate handlers, you will need to go to into IIS and navigate to the handler mappings for the affected domain. In this window, you will want to Revert to parent. After the handler mappings have been reset, you should now be able to go back into Plesk and disable/re-enable PHP to resolve the issue.