Since it's already been answered, I'm just gonna make an example where that particular meaning of そんな could be used.

> A: The truth is that I'm not your real father...

> B: そんな。。。

And you could imagine the shocked child falling to his/her knees.

That そんな can be translated as: "It cannot be..." or "Impossible..." or "No way...". As I see it, it's close to what Nothing at all commented.