Cancelling the plan that includes Portfolio eventually takes your site offline, but it does not delete your work.
Your live site
Your site does not come down immediately. It enters a grace period that keeps it live for 14 days after the cancellation. After that, your site is unpublished and visitors to your myportfolio.com address or custom domain will no longer see it.
Your content
Your pages, projects, images, text, and theme settings are kept on your Adobe account. There is no time limit on this - your content is not deleted.
You can still sign in to the editor at portfolio.adobe.com without an active plan, and keep working on your site. Publishing is the part that needs a plan, so to make your site live again you need a plan that includes Portfolio.
Your custom domain
Your domain stays registered with the company you bought it from and is unaffected by cancelling your Adobe plan. Because your site is no longer published, the domain will not show your site until you publish again. If you bought the domain through Portfolio, it is registered with Namecheap and renews separately.
Coming back
See Reactivate a lapsed account and restore your site. Note that your site may need to be moved back to the profile holding your new plan - renewing alone does not always restore it.