expression
what's wrong?; did something happen?; what's up?
Casual, often concerned or curious question when someone seems upset, distracted, or different from usual. The の ending adds an explanatory or soft, feminine tone.
元気ないね。何かあったの？
You seem down. Did something happen?
急に黙っちゃって、何かあったの？
You suddenly went quiet. What's wrong?