Formal:
* 發生咗咩事？ (faat sing zo mei sei?) - Literally translates to "What happened?" This is the most formal way to ask.
Informal:
* 點解？ (dim gai?) - This means "Why?" but can also be used to ask "What happened?" in informal contexts.
* 做乜？ (zou mat?) - This literally translates to "Do what?", but it's a very common way to ask "What happened?" in everyday conversations.
* 點呀？ (dim aa?) - This means "How is it?" but can also be used to ask "What happened?" in informal contexts.
Example:
* 你點解會跌倒？ (nei dim gai wui dit dou?) - Why did you fall?
* 你做乜咁唔開心？ (nei zou mat gam bat hoi sam?) - What happened? Why are you so unhappy?
It's important to consider the context and the level of formality when choosing the right phrase. You can also use different tones and facial expressions to convey your meaning.