My Oakley horsebox seems to have isolated itself and wont start?

I have just bought it. I charged the battery as it didnt have enough power to start it, although it did try. The next day i tried again but now it is as though an isolator switch has been turned. Therer is no power at all, no lights, nothing. The box has a sign in the window which says Bell security and a key fob but this doesnt seem to do anything. Not sure if this is the reason???