。
She was the sole survivor of that crash.
Literal
She [topic-は] that crash accident [genitive-の] sole [genitive-の] survivor was.
墜落事故 ('crash accident') is the standard term for an aviation or falling-object disaster — 墜落 specifically denotes falling-from-height crashes, distinct from 衝突 ('collision') or 接触 ('contact incident'). 唯一 ('sole, only') is more formal than だけ — it precedes a noun directly, often with の (唯一の). 生存者 is built from 生存 ('survival') + ~者 (an agent suffix forming person-words). であった is the formal/literary past of だ, slightly stiffer than だった — common in news writing and formal narration. The whole phrasing has the gravity of a news report.