"What happen" is not a grammatically correct phrase in English. The correct phrase is "What happened?" which means "Apa yang terjadi?" in Indonesian. It is used to ask about a past event or situation. Another similar phrase is "Whats going on?" which also means "Apa yang terjadi?" in Indonesian. "What did happen?" is also a correct phrase in English, but it is usually used to confirm the truth of a past event.