'happened' is an example of past tense or past participle verb, which describes an action that has already taken place.
the rules and use cases associated with this verb requires that the subject of the verb is placed before the verb, which in this word is 'has'. In other words, the verb should always be preceded by 'has'.
1. 'He has happened upon the scene.'
2. 'The accident has happened while they were walking.'
3. 'It has happened before.'
avoid using the verb 'happened' when you mean 'to occur'. Although many people use 'happen' and 'occur' interchangeably, they actually hold different meanings. While happen is more related to an unexpected or unplanned event, 'occur' is more related to a predictable event.