1. Used alone in response to your addressee's comments to mean that you do not believe what they say is true, or possible, or that it really surprises you. English = 'I don't believe it!', 'It can't be (true)!', 'Really!?', and so on.
2. Used alone in response to an event to mean that you find it difficult to accept or believe. You are surprised. English = 'It isn't possible!', 'It can't be (true)!', and so on.