Non Responsive – Suddenly stopped working! :(

Please can you assist. I simply logged into the WP-admin and navigated around the cms to get a feel for the theme. I did not make any changes however it appears the mobile site has stop being responsive. If I change my browser size it does not alter and when viewed on my iphone it simple shows the site in full desktop mode but the menu and header is not displaying correctly. Like i said however i have not made any changes to the css or header. Can someone please advise as on the face of it, it looks like a logged into the cms and now it does not work correctly. Any help would be appreciated. Thank you.