"Nothing has happened."
Translation:No ha ocurrido nada.
Technically you need the no. Any negative like nunca or nada relating to a verb needs a no as well.
Reading this, it looks like it doesn't have to be a double negative as long as "nada" is used at the beginning... but Duo doesn't like it.
My translation: "Nada ha ocurrido". I think it is correct, too. How do you think?