。
Years of effort went up in smoke.
Literal
Long-years [possessive-の] effort [subject-が] nothingness [to-に] returned.
烏有に帰す (うゆうにきす, 'to come to nothing, to be reduced to ashes') is a literary idiom from Chinese: 烏有 means 'crow-existence' = 'nonexistence' (a thing as absent as a crow — there is nothing). 長年の努力 ('years of effort') is the thing lost. The sentence is a concise, devastating lament using classical vocabulary.