This forum was down today - and possibly some of yesterday.
Our domain provider updated their system some time ago and the technical and billing contacts that the renewal notices get sent to reverted to an email address we first signed up with but no longer use. So when it came time for the domain renewal we didn't get the notification.
I've renewed the domain name for the next 5 years (and updated our contact info there) so we should be good to go until at least then.
Sorry for the inconvenience - I should have been more on top of this.
Thanks also to Kevin for letting us know.