Sorry, that file cannot be edited.

Sorry, that file cannot be edited.

That is the error I get when I press editor under the appearance tab in the admin area.
It was fine yesterday but today nothing, I’ve installed various themes but still get same error. I’ve changed the permission of theme_editor.php to 777 but that just gives a 404 error when editor is pressed.

I’m a newbie at Wordpres so maybe it was something I did.

Any help would be great