This problem is caused when you are using a non-standard index file for your site's index.

For example Default.htm is your site's index file. In this case you have to log in your cPanel and go to the File Manager. Navigate to the public_html directory and create a new file called .htaccess. In this file specify:

DirectoryIndex Default.htm

This Apache directive will instruct the webserver to open the file Default.htm when your site is opened.