This is the 8836th most frequent Russian word.
потерянные
"Lost"
In this sentence, "потерянные" is used as an adjective describing things (вещи) that had been lost, forming a participle from the verb "терять".
Потерянные вещи были найдены через несколько часов.
The lost items were found after a few hours.
Here, "потерянным" is a form of the adjective "потерянный" used in its singular masculine instrumental case, indicating a personal state of feeling lost or bewildered.
Он чувствовал себя потерянным после того, как переехал в новый город.
He felt lost after moving to a new city.
"Потерянные" acts as a descriptive adjective for "документы," referring to papers or files that have been lost.