If you recently canceled your Google Fi service, and haven't transferred your number to another carrier, you can undo your cancellation from the Google Fi app or website for up to 7 days. Otherwise, you can contact Google Fi Support to re-activate your account for up to 60 days after cancellation.
If your Fi account was canceled more than 60 days ago, you can open a new Fi account directly from the Google Fi website.
If your Fi account was closed for non-payment, you can contact Google Fi Support to pay your outstanding balance and open a new Fi account.