If you've set up Let's Encrypt certificates in the past, you would've most likely received emails indicating that action is required in order to prevent upcoming renewals from breaking (starting Feb. 13, 2019).
Depending on your deployment, you can test against their staging environment which has TLS-SNI-01 disabled here: https://letsencrypt.org/docs/staging-environment/. Most people would use Certbot for ease of installation and some work is required to keep it running without issues. Steps are as follows:
1. Check if Certbot is 0.28 or newer
certbot --version || /path/to/certbot-auto --version
2. If it's older than 0.28 then follow instructions on https://certbot.eff.org/ to update
3. Remove references to
tls-sni-01 in your configuration
sudo sh -c "sed -i.bak -e 's/^\(pref_challs.*\)tls-sni-01\(.*\)/\1http-01\2/g' /etc/letsencrypt/renewal/*; rm -f /etc/letsencrypt/renewal/*.bak"
4. Do a dry run to test
sudo certbot renew --dry-run
If the dry run is successful and Certbot is 0.28 or higher then all is well. If there are any failures, fix the errors and rerun. Sometimes errors could be server availability especially for their staging environment. You can see status information here: https://letsencrypt.status.io/