Each server has a list of default files it will server up as default if a file isnt specified. If index.php isnt being served up then either one of the file types that is a higher on the list exists or index.php isnt on the list. Most servers allow you to modify the default files list via the control panel.
I think you can use .htaccess to set your default page. Create .htaccess (with the dot) Then copy and paste this in the file DirectoryIndex index.php
Put a index.htm or index.html file and see if that works, if it does, you can redirect that to the index.php Like some of the guys already said above,contact your hosting provider.You can also set a redirect in your apache conf file if you know how to do it.