すみません、駅のコインロッカーの鍵をなくしてしまいました。 — Excuse me, I lost the key to a coin locker at the station. (Train · Station, Formal — to staff, JLPT N4)

Gesture & etiquette

Point towards the general direction of the lockers, or make a gesture of searching your pockets.

Do not attempt to force open any locker. Provide as much detail as possible (location, time, locker number if remembered, contents) to the station staff.