Let's say someone calls computer support for the first time on a Monday, and reaches the internal support department at their company.
Hello, Desktop Support.
--Hi, this is Anthony in Accounting. I've got a problem with my PC. It just stopped working.
Anthony, call you tell me what happened?
Support wants to know what took place at the moment the PC stopped working.
Now let's say Anthony calls Desktop Support again on Tuesday, with a different problem. And he calls again on Wednesday and again on Thursday with yet other problems.
On Friday, he calls again. This time, Desktop Support, who have come to recognize Anthony as a regular caller, and a person who seems to have very bad luck with computers, might ask:
Hello, Anthony. What's happened now?
Meaning, what is this most recent trouble in a series of troubles that you're calling us about?
Or someone at a coffee shop might ask "Have you seen the news on TV??!!" to which you might reply,
Why, what's happened?
meaning, what is this breaking news I should know about?
The present perfect casts the action in some present light.