LetsEncrypt allows you to install free SSL certificates for your website.
Over time LetsEncrypt have upgraded and changed the way it works for security reasons, if you have older versions of LetsEncrypt on your server it could cause it from failing to renew your certificate.
Below are commands you can use to uprade your version of certbot which enables you to upgrade your certificate.
apt-get update apt-get install software-properties-common add-apt-repository universe add-apt-repository ppa:certbot/certbot apt-get update apt-get install certbot python3-certbot-nginx
Then you can simply run
certbot you'll be shown a list of all websites you can choose from to assign a SSL certificate for, select the site you want to renew and LetsEncrypt to assign a new certificate for your website.