A different situation. "What happened" can be asked when you see something has changed. ex; You walk into your friends room and it is very messy. "Geez, what happened?" "What's the matter", or "what's wrong", are used when you know something bad has happened. ex; You come upon your friend crying, "What's the matter?" The key difference is what you are addressing, if something is clearly wrong you could use either but "what's the matter" will only be used in the second kind of situation.