A casual question asking if something has happened; used to check if someone is upset or something is wrong (informal, often to someone you know).
顔色が悪いね。何かあったの？
You look unwell. Did something happen?
電車でずっと下を向いていた友達に『何かあったの』と声をかけた。
I asked my friend, who had been looking down on the train, 'Is something the matter?'
試験の結果を見て、母はやさしく『何かあったの？』と聞いた。
Seeing the exam results, my mother gently asked, 'Did something happen?'
部長が急に厳しい顔になったので、同僚が『何かあったの？』と尋ねた。
The manager suddenly looked stern, so a colleague asked, 'Is something wrong?'