。
She lost her memory in a traffic accident.
Literal
She [topic-は] traffic-accident [by-で] memory [object-を] lost.
事故で ('in / due to an accident') uses で as the causal/instrumental marker — the accident is the cause of the loss. 記憶を失う ('to lose one's memory') is the standard collocation; 失う ('to lose, be deprived of') is more formal than なくす ('to lose [physically]'). Memory loss after a traffic accident is, alas, a common dramatic plot element in Japanese television and manga; in real life, it's a recognized post-trauma phenomenon (記憶喪失, retrograde amnesia).