。
She died in a car accident.
Literal
She [topic-は] car-accident [in-で] passed-away.
亡くなる is the standard polite/respectful word for 'to pass away,' the everyday euphemism that replaces the blunt 死ぬ ('to die') in news, obituaries, and conversations about anyone the speaker knew or respects. 死ぬ used of a real person can sound jarring or callous; 亡くなる is the default choice. The で here marks the cause or circumstance ('died in/from a car accident'), parallel to the で in 病気で死ぬ ('die of illness'). 自動車事故 is a transparent compound: 自動車 ('automobile') + 事故 ('accident').