Even if photosynthesis is blocked, Mankind will access a huge amount of dying and dead biomass, complementing gas and oil.
Fungi might provide some nutrients. Zero calorie as I understand, but they provide essential metals. Alas, they concentrate radioactivity too, needing spared locations.
Grass and wood to human food isn't immediate, but for instance methanol from pyrolysis can become pentaerythrose and pentaerythritol, or ethylene and then fatty acids and so on.