This happens because Recurring Emails for Gmail is no longer authorized to send emails on your behalf. This can happen for several reasons and one of the most common causes is changing your Google password which in turn causes the revoking of your apps' permissions. This is a security feature implemented by Google. To fix it, log in and grant the necessary permissions again via the My Emails page or via the mail.google.com interface -> Send Later button and schedule the email again. Your email is not lost and cancelled emails can be enabled by editing and saving them in My Emails.