I am glad your other games are still working.
Are you running any mods? As a unique metadata.json file is in each one. If the metadata.json is missing it would be missing for all. The only thing I can think of was there was corruption in the save file referencing the metadata.json.
So there would be the metadata.json for the stoneheath.smod, rayyas_children.smod and rayyas_children_ui.smod for the base game. Each one would have a metadata.json which identifies the file location, mixins and language.
Someone better versed would have to address this issue as I am stumped.
@8BitCrab have you seen this issue before. It is only effecting a single save game.