It is possible you have received the Let's Encrypt warning e-mail with the message:
Your Let's Encrypt client used ACME TLS-SNI-01 domain validation to issue a certificate in the past 60 day. Action may be required to prevent your Let's Encrypt certificate renewals from breaking.
In most cased this means that the installed letsencrypt version is outdated. This is the case for Ubuntu 16.04.
Follow these steps to upgrade from the letsencrypt tool to the certbot tool. This will install the latest certbot version from a PPA archive maintained by the Let's Encrypt team. You can still use the letsencrypt command line because a symlink is created.
sudo apt-get update sudo apt-get install software-properties-common sudo add-apt-repository universe sudo add-apt-repository ppa:certbot/certbot sudo apt-get update sudo apt-get install certbot python-certbot-apache
You can test the automatic renewal with the dry-run option
sudo certbot renew --dry-run