There's some information on expiration emails in this thread:
And in the documentation:
Usually, people are confused when they get an email because they recently changed something about the certificate (adding or removing a domain name), so the original (without the change) is expiring without an exact replacement, so you're alerted but there really isn't anything wrong. But if your certificate actually is expiring soon, then you need to figure out why.
If you are on a "shared hosting" plan, where you don't administrate the web server directly yourself but only through some kind of control panel, then you'd need to talk to your hosting company since they should be handling the certificates for you. If you do administrate the web server yourself, then everything depends on what software was installed (the ACME client), so you need to understand which one that was (certbot, for instance, is a popular choice) and look at its logs to figure out why it's failing to renew.
But without more information from you (like the answers to the questions above), it's really hard to give you any more specific advice than that. There are a bazillion ways a system might be set up to get certificates, and almost as many reasons that it might be failing.