The Opposite of Different Languages

Is there an example of where two languages historically considered different languages by the people that speak them suddenly become a single language? Well, I think that what comes to my mind is Arabic. With English, you don't quite have that, you know, with standard British English or American English. There isn't one kind of master standard English; it's more to do with the actual use of the language and which can be represented diagrammatically in this kind of mega-ven diagram.