Files not writable in the Dashboard

Hi . I was just trying to edit the style.css file from w/in wordpress and noticed that where there is normally an “Update” button, there is a message, “You need to make this file writable before you can save your changes.”

Why did I lose the “Udpate” button and how do I get it back? This message is showing on all editable files (index.php,single.php, sidebar.php, style.css etc etc)