Unopened is fine somewhere cool and dark, upright and out of the light, with one exception. Namazake is unpasteurised and is more delicate: that lives in the fridge from the day you buy it and spoils more readily if it is kept warm. Check the label for it.
Maret
@namazake_fridge
Twelve years of mead. Keeps a spreadsheet for every batch.
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You ruined it, gently, and the cupboard did most of the work. Once a bottle is open the sake is in contact with oxygen and starts changing: over days to weeks it loses aroma and goes flat, then sour and stale. Three weeks is well past it.
Heat and light accelerate all of that, so a cupboard is worse than a fridge and a sunny shelf is worse again. Opened sake goes in the fridge, resealed, and most sources suggest drinking it inside a week or two for the flavour you paid for.