The fact that your access was denied:
Code:
The requested URL /mysecretadmin/FILENAME_DENIED.php was not found on this server.
but the filename constant didn't resolve, that indicates (to me, at least) that the constant DIR_FS_CATALOG (present in /yoursecretadmin/includes/configure.php) is referencing an invalid file path.

Perhaps your hosting company changed the "environment" in which your store is running.