It means that she was not available to men for romantic or sexual purposes.

It means like, even though men wanted her, she was unattainable. As in, it was very difficult to establish a relationship with her. It's kind of related to the phrase "to play hard to get," where a woman acts disinterested in a man so that he may put more effort into "getting" her. This comes from the idea that men enjoy chasing after women and are more likely to cherish those who are not "easy." In this case, the woman was indeed hard to get, until that one explosive day.