Can I say "What did happen" instead?
"What did happen" is a question that emphasizes the "did" aspect (it is similar to the question "what actually happened"), so it would be more suitable to ask in situations when real events need to be clarified or concerned. When asking in a general way, "what happened" is the most appropriate expression to use. Ex: Oh my lord, this house is a mess. What happened while I was out? Ex: What happened? Tell me why you're upset.