How to troubleshoot "digital signature is invalid" message at customer?

We purchased a three-year code signing certificate two years ago. Today I have a single case of a customer receiving a message while running our software: “warning, the digital signature is invalid and cannot be trusted”, et cetera.

Could anyone give some possible causes for an isolated incident like this? I presume that our certificate is actually still valid because our user base is fairly large and still active and we only have this single incident, but I’d like to respond to our one customer with some potential causes.

Thank you,

Time and date on their computer is the usual suspect.