What happens to my iCloud files if I can't renew my subscription?

Click on the link to read about "Downgrade or cancel your iCloud storage plan" on an iOS device, a Mac, or a PC. --> https://support.apple.com/HT207594

• "If you downgrade to the free iCloud plan, your storage will be reduced to 5GB and you will no longer have access to iCloud+ features like Hide My Email, Private Relay, and HomeKit Secure Video support.*"

https://www.apple.com/legal/internet-services/icloud/en/terms.html

III A. "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."

Anything over 5GB will be deleted. So if you know you won't be able to cover the payment, download and backup all the data on iCloud before the current subscription period ends.