。
She had lost her parents in a plane crash.
Literal
Parents [object-を] airplane-accident [in-で] lost [explanatory-のだった].
なくす here means 'to lose someone to death' — more emotionally loaded than 失う. のだった (past form of のだ) adds an explanatory narrative nuance — 'it was the case that...' — common in storytelling to provide backstory. The subject is omitted.