What could happen to my iCloud+ storage if I didn’t pay?

The terms spell out what may happen -> Legal - iCloud - Apple

“The applicable fee for an upgraded plan will take effect immediately; downgrades to your 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 of your selected plan.”

Any data that exceeds the free 5GB option will be deleted. Make sure you either fix the issue and have it successfully charge you, or backup your data before it gets deleted.