？
What's wrong?
Literal
What [subject-が] problem [copula-な-explanatory-question-の]?
問題 is a noun ('problem'), so the explanatory ending takes the form なの (where な is the form of だ used before の). The full structure is [noun]+な+の, the standard pattern for nominal-predicate questions in casual speech. A common conversational way to ask 'what's wrong / what's the matter?'