What happened? / What’s up? / What’s the matter? / What’s going on?

ANSWER:“What’s up?” can be used in two ways. One common way is as an informal greeting to mean “hello” or “how are you?” We use this with friends or people we know well in daily situations.

Jeff: What’s up?
Lucy: Not much. How about you?

We also use “What’s up?” when we ask what the situation is at a specific time. We use “What’s going on?” for the same purpose. For example, if you return home and there are police cars in front of your neighbor’s house, you may ask your husband or wife, “What’s up?” or “What’s going on?” You want to get information about this specific situation.

“What’s the matter?” and “What happened?” are also used to ask for information about a specific situation. However, these two questions are normally used for a situation where something bad or negative has happened or may have happened. For example, when your 6-year-old daughter starts crying, you may ask her, “What’s the matter?” and/or “What happened?” If your co-worker walks into your office with an angry look on her face, you may ask, “What’s the matter?” and/or “What happened?”

You can also use “What happened?” to ask about the result of something. If you know your co-worker asked your boss for a raise (higher pay or salary) and you see your co-worker the next day, you may ask, “What happened?” to find out the result.
Thanks for the question, Raul, and I hope this is helpful.