Q: what happens to iCloud storage if can't pay one month

It'll revert to the free 5 GB plan --  Get help using iCloud storage - Apple Support & iCloud storage upgrades and downgrades - Apple Support

As far as anyone knows, though there may be a short 'grace period', if you fail to pay the subscription then all data falling outside the free 5GB will be deleted, and there's no way of knowing exactly which data it will be. It presumably won't be available when you re-subscribe (I don't know whether again the allow a short grace period to allow for accidents but it wouldn't be of any significant length).