。
She lost her car key.
Literal
She [topic-は] car [of-の] key [object-を] lost.
キー is the English loanword 'key,' used interchangeably with the Japanese 鍵 (かぎ). なくした is the past tense of なくす (to lose something) — a common everyday verb for misplacing items. The sentence is minimal and practical, the kind of statement that comes up constantly in daily life.