What Was Lost is the 2007 début novel by Catherine O'Flynn. The novel is about a girl who goes missing in a shopping centre in 1984, and the people who try to discover what happened to her twenty years later. What Was Lost won the First Novel Award at the 2007 Costa Book Awards. The Costa Book Awards' judging panel, praised the novel for ""blending humour and pathos in a cleverly constructed and absorbing mystery."" They described the novel as inventive, compelling, and poignant.