出事(chū shì) - to have an accident, for something to go wrong in Mandarin

出事 (chū shì) is the everyday way to say that something has gone badly wrong: an accident, an injury, a disaster. Literally the two characters only say that a matter has arisen, but nobody uses it for good news.

The trap is that the pieces look neutral. 事 is just an affair, and a beginner who builds 出事 out of the parts expects "something came up". It does not mean that; for a diary clash you want 有事 (yǒu shì). 出事 means the ambulance is on its way.

It is also a verb-object compound, which means it splits. 出了事 puts the aspect marker in the middle, and 出了什么事？ is the natural way to ask what happened. Treating it as one inseparable block will make your sentences come out wrong.