The general warning browsers show when a certificate check fails.
WHAT IT MEANS
One of the three checks failed: the issuer is not trusted, the certificate is expired, or it does not cover this hostname.
FINDING THE SPECIFIC REASON
Click Advanced on the warning page. Browsers show a specific error code beneath the general message. That code identifies the cause precisely.
The common ones are covered in their own articles here: date invalid, common name invalid, authority invalid.
WHAT VISITORS DO
They leave. A very small proportion click through the warning. Treat this as the site being down.
FIRST CHECKS
Is the certificate expired? Click the padlock area and view the certificate dates. Does the hostname match? Visiting www when the certificate covers only the bare domain produces this. Did you recently change hosting or DNS?
IF IT AFFECTS ONLY YOU
Check your computer's date and time. A clock badly out of step makes every certificate appear invalid.
IF IT AFFECTS EVERYONE
Run AutoSSL and check cPanel > SSL/TLS Status.
