If it is, uninstall it and try installing it again.
It is Wordpress which will use the "index.php" file, but I think some other files and / or directories may be missing.
Is this a new installation? If it is, uninstall it and try installing it again.
NO
There is a problem on your server with PHP. It has been passed on to the Admins. Whether they will fix it before or after the migration of accounts is done, I do not know.