This is the 944th most frequent Russian word.
произошедшего
"Of what happened" or "that occurred."
Here, 'произошедшего' is used in the genitive case as the object of 'забыть', referring to an event that occurred.
Он не может забыть произошедшего.
He cannot forget what happened.
In this sentence, 'произошедшего' is used in the genitive case to describe 'reason', directly attributing the incident that took place.
Расследование выявило причину произошедшего.
The investigation revealed the cause of the incident.
Here, 'произошедшего' also appears in the genitive case, indicating a source (from what happened) from which a lesson is to be learned.