Ant222,
In natural English, there are unspoken rules which trigger certain repsonse when certain questions are asked.
If you apply the same question to a person in the following scenarios, this is probably what it will sound like:
Adam walked into the office area. He saw a bunch of co-workers hovering over a desk.
Adam: What happened! He leaned over the crowd.
John: Mary just passed out on her desk. I was jsut discussing the project with her on the phone in my office.
In this case, simple past is just fine.
Scenario 2
The same question applied to a situation or inanimate object:
You arrive the at the building where your office is located. A fire barricade has been set up and water is everywhere.
You asked the fireman: What happened!
Fireman responded: The offices on the entire 10th floor have been destroyed by a quickly moving fire and the building has been evacuted. Simple past will be understood, though not very idiomatic.
In this type of scenarios, present perfect is naturally preferred.