What is the most natural translation of the English phrase, "What's wrong?" (i.e. what you would ask a person who appears sad or hurt or withdrawn)? ¿Qué te pasó? would be one option, but that seems like a blunt "What wrong with you?" or "What happened to you?" instead of a compassionate "What's wrong?"
Most common way that I hear in Argentina is
¿Qué (te) pasa?
This can be used for the situation you described, as a compassionate "What's wrong?" But there are heaps of other ways that could be used just as well. The feeling for each is really found in the tone in your voice when you ask the question.
¿Qué pasó? - What happened?
¿Cuál es el problema? - What is the problem?
¿Qué hay? - What's the matter?
¿Pasa algo? - is something the matter?
This one feels a little less compassionate imo
¿Qué tienes? - What's your problem?