I don't know what's wrong with him/herUsed to express uncertainty about someone's behavior, condition, or situation. The pronoun can change: no sé qué les pasa means “I don't know what's wrong with them,” and no sé qué le pasó refers to what happened to him or her.
SP: No sé qué le pasa a Marta; está muy callada hoy.
EN: I don't know what's wrong with Marta; she's very quiet today.