A live site has suddenly stopped populating with the Page.ss template ( theme/templates/Layout/Page.ss). Pages with custom templates are fine but page page types won't display any data. Therefore there is no log in form for the admin either. No changes have been made to the files. I'm unsure as to what has been changed via the CMS but you can't view the admin form any longer to log in. I've tried replacing the Page.ss file to the original default one, removing extra JS but no luck. I'm stuck as to how to trouble shoot this?
The hosting had run out of space. This was causing the described behaviour. Not a SilverStripe issue or site issue but a hosting issue. I'll leave this question / answer up as it may help someone puzzled in the future.