Error 403 (Forbidden, access denied) can occur for one of several reasons:
You have uploaded the wrong index file to the server or it is missing. Your site's home page file should be located in the site directory and named index.html, index.htm, or index.php. Please note that all characters must be lowercase (Unlike Windows, the Linux OS that runs the servers does matter the case of file names, and index.html and Index.html are two completely different files).
Incorrect permissions have been set for the directory in which the requested file is located, or for any of its parent directories. All of these directories must provide at least read (r) and execute (x) permissions to the owner.
The site files are placed in the wrong directory. The correct path to the site directory can be seen in the "Directory" column, section "Sites-domains" in the Plesk hosting control panel.