If I stop paying for the professional plan would I lose all my files or they are just going to be locked?

Hey @Cesar_Cossio ,
Welcome to the community and for reaching out to us!
When you downgrade you only lose the professional features. All of your files and projects will remain. When you upgrade again the features will return.

As a reminder, on a Starter plan, your team will have one project. If your team has more than one project when you downgrade, Figma will lock your team and prevent you from editing your files. To keep editing, you’ll need to reorganize your files and projects so they are under those limits. Learn more about moving projects and moving files.
For more information when you cancel your Professional plan subscription, you can check our guide: here Hope this helps!