occur
The German word 'geschehen' is used to describe something that happens or occurs. It is often used in the context of events or situations that take place. For example, one might say 'Was ist geschehen?' to ask 'What happened?' or 'Das Unglück ist geschehen' to say 'The accident occurred.'
What has happened?
This is a question in past tense, asking about an event or situation that has occurred.
Something terrible must have happened.
This is an assumption or conclusion that something unpleasant took place.
That cannot have happened.
This is a statement of disbelief regarding a past event.