。
I suddenly became sad.
Literal
I [topic-は] suddenly sad became.
急に (きゅうに, 'suddenly') — the な-adjective 急 in adverbial に form. 悲しくなった — the い-adjective 悲しい in く form + なる ('become'). The く-なる pattern is how い-adjectives express change of state.
。
I suddenly became sad.