。
On the flip side of sorrow, there is joy.
Literal
Sorrow-of opposite-side-[on-には] joy [subject-が] exist-[nominalizer-ものだ].
Aphoristic register. 反面 ('the other side, flip side') is a useful noun for juxtaposition — 'on the other hand, on the flip side.' ものだ at the end of a sentence can mark a general truth or universal observation — 'it's the way things are.' 憂い ('sorrow, grief') is a slightly poetic noun. The whole sentence reads like a piece of proverbial wisdom.