Check the lifetime first. A certificate with around ninety days of life is automated. One with a year or more was bought and installed by hand. The two situations need different responses.
If it is automated
An expiry warning means the renewal has stopped running, and the certificate is simply where you noticed. Renewing it by hand and moving on means it happens again in ninety days.
The task is finding where the renewal actually runs: a timer on a server, a hosting control panel, a proxy, a platform handling it invisibly. Common cause is a server migration that left the job behind.
If it was installed manually
Replace it before it expires, install the intermediate certificate alongside it, and check every name the old one covered is on the new one.
What happens if it lapses
Browsers show a full-page warning before your site loads. Most visitors leave at that screen. API clients and payment integrations fail outright rather than warning.
This is the short version. The full guide covers it properly: read more.