Do you have anybody to help you with the running of your server? It would help to be familiar with how it is setup.
It looks like somebody already set Let’s Encrypt up originally but it is failing to renew.
You will need to SSH in to your server and run letsencrypt renew, certbot renew, or certbot-auto renew, depending on what command was originally used. This will try to renew the certificate and reveal what the issue may be.
There is also a chance that Certbot was not used, but only the person who set it up originally would know the answer.
Additional context: there was an issue with Let’s Encrypt recently which may be the reason for the failing renewal, and that thread provides some advice on what to do.