My Ford S-Max won't start?

I've got a five year old ford s-max. I had just driven back home and turned the engine off momentarily. When I tried to turn the engine on again it would not start, only a sharp clicking sound when I turned the key. Windows would not go up either. We managed to push start the car and it sounded alright however once the engine was turned off it went back to square one. Do you reckon it might simply be the battery which needs changing (I've never changed the battery since I bought it , new) or something more sinister perhaps? Thanks