。
She lost interest in her work.
Literal
She [topic-は] work [target-に] interest [object-を] lost.
興味を失う ('lose interest') is the standard verbal collocation, with 興味 ('interest, curiosity') as the object of losing. The に marks what the interest was directed toward — different from English 'in,' which feels like a passive container; Japanese frames the interest as oriented *to* the target. The opposite is 興味を持つ ('have interest, be interested in'), the standard frame for picking up a new interest.