。
She pressed a piano key.
Literal
She [topic-は] piano [of-の] key [object-を] pressed.
キー is a routine English borrowing — used here for piano keys, but also for keyboard keys (computer keys), keys to a lock, and the 'key' of a song. The native Japanese word 鍵 (かぎ) covers 'key to a lock' and metaphorical 'key to a problem,' while 鍵盤 (けんばん) refers to a keyboard as a whole — overlapping but not identical territory. Where English speakers might say 'press a key,' Japanese has the same construction: キーを押す.