What happens to your data when iCloud downgrades the storage plan?

Hello!!!

Let's say you have the 500GB iCloud storage plan and you are using 220GB of that total space. Assuming you are paying the monthly storage fee, what happens when the transaction fails and it goes back to 5GB? (i.e. the card is declined, an error occurred with the billing information, or you get a different credit card, and the old one becomes inactive) Do you lose your data in the cloud, or does it simply become impossible to upload more data?

Thank you so much in advance!!!

Alan