A casual phrase meaning 'What's the matter?' or 'What's wrong?' used to ask about someone's condition or situation.
顔色が悪いよ。どうしたの？
You look pale. What's wrong?
急に泣き出したけど、どうしたの？
You suddenly started crying — what's the matter?
遅刻したね。どうしたの？
You're late. What happened?
Used to ask 'what happened to...' or 'where is...' in patterns like '…をどうしたの?' or '…はどうしたの?'.
財布はどうしたの？見当たらないけど。
What happened to your wallet? I can't find it.
昨日の宿題はどうしたの？提出した？
What happened to yesterday's homework? Did you turn it in?