You should read the terms of use carefully -> https://www.apple.com/legal/internet-services/icloud/. Especially the section quoted below:
”The applicable storage fee for an upgraded storage plan will take effect immediately; downgrades to your storage plan will take effect on the next annual or monthly billing date. YOU ARE RESPONSIBLE FOR THE TIMELY PAYMENT OF ALL FEES AND FOR PROVIDING APPLE WITH VALID CREDIT CARD OR PAYMENT ACCOUNT DETAILS FOR PAYMENT OF ALL FEES. If Apple is unable to successfully charge your credit card or payment account for fees due, Apple reserves the right to revoke or restrict access to your stored Content, delete your stored Content, or terminate your Account. If you want to designate a different credit card or payment account or if there is a change in your credit card or payment account status, you must change your information online in the Account Information section of iCloud; this may temporarily disrupt your access to the Services while Apple verifies your new payment information. We may contact you via email regarding your account, for reasons including, without limitation, reaching or exceeding your storage limit.”
You’re renting storage. If your data is valuable to you, either don’t miss payments for that extra space, or cancel the subscription and backup your data somewhere else if you cannot pay for it.