This is the 17900th most frequent Russian word.
утерянного
"Lost" or "of something lost."
Here, 'утерянного' describes the time ('времени') that has been lost, showing a sense of irretrievable absence.
Нам нужно вернуть ценность утерянного времени.
We need to restore the value of the lost time.
In this sentence, 'утерянного' qualifies the treasure ('сокровища') and implies that it is missing or no longer possessed.
Он рассказал историю утерянного сокровища.
He told the story of the lost treasure.
This usage of 'утерянного' relates to the umbrella ('зонта') that has been left behind or misplaced.