Riven arrived at the library eighty years before the events of A Court of Silver Flames. The events that led to her arriving at the library are unknown, but the result is that she is uncomfortable with any sort of contact with strangers. She doesn't speak of what happened to her. Only she, Clotho and Rhysand know what happened to her.
A Court of Silver Flames
Riven is seen when Gwyn comes to thank Nesta for swapping out the books from Merril. She looks at the two of them, and then scurries off. Gwyn explains that Riven arrived at the library eighty years ago, and that she doesn't know what happened to her, and that their stories remain their own to tell, and that Clotho and Rhysand are the only others who know what happened to her. She also tells Nesta that she doesn't know if Riven has utilized the resources available to priestesses in the library.