index.php permissions keeps resetting to 000

Something has happened to my index.php file that caused it to keep resetting permissions to 000. This causes a 403 Forbidden message to appear when I access any page on my blog.

I’ve tried resetting permissions in the File Manager but they won’t change.
I’ve tried deleting the index.php file but it keeps reappearing.
I’ve tried replacing the index file and it won’t be replaced.

I can still access my Dashboard, so this affects only the visible area of the blog.

Does anyone have a solution for this?
Thanks.