Behind every misfortune lies a blessing.

Literal

Misfortune's reverse [at-に] [topic-は] fortune exists.

不幸の裏には幸いあり — a Japanese proverb: 'behind misfortune is good fortune.' 裏 = 'back/reverse.' あり is a classical/proverbial form of ある. Similar to 'every cloud has a silver lining.'