Do not panic, If you are getting this error first answer the following questions:
- Is this error happening only in a few computers and not in all devices?
- Is the error happing only on Windows XP?
- Is the error happening on Chrome?
If you said yes to all the previous questions, then this is because you are using an SSL certificate that is not compatible because it is newer and more secure than the computers gving the error, the certificate version that you are using didn't exist when Windows XP was around, and since Microsoft and even Google are not providing any more updates, it is not able to use the certificate, it is as trying to use a VHS player to play a DVD.
From Is Windows XP secure?:
"Windows XP is dead for Microsoft, they are not sending any more security updates or any updates at all as a matter of fact, by not having new updates many other software companies decided not to create updates as well for users on Windows XP. Chrome is one of them and starting April 2016, Chrome will continue to function on these platforms but will no longer receive updates and security fixes."
Conclusion: don't worry, your site is secure, but the computer that you are using is not secure and needs to be updated, or changed ASAP.