What happens to Nagaina at the end of the story?

I think you're referring to the story "Rikki-Tikki-Tavi" by Rudyard Kipling.

Nagaina goes down the rat hole (her home) with Rikki (the mongoose) clenched to her tail. When he comes out of the hole, he says that he's killed her. The ants march down to check and later the tailor bird announces it to the whole garden.