I’m assuming it’s a Fully Hosted domain, as evidenced by the existence of an index.html file
Look for a .htaccess file in the same directory as index.html. It’d be a hidden file, so your FTP client needs to be set to display hidden files.
For now, if you don’t know how to edit the contents of the .htaccess file, just rename it to .htaccess.old
If that breaks the site, but at least gets your index.html page working, post the contents of .htaccess, minus any sensitive information, and we’ll take a look.