When people access a site with an expired SSL/TLS certificate, they see a warning, telling them the site may no longer be secure. The advice for consumers: if this warning pops up, don’t proceed to the site in question because any data shared may be vulnerable to interception. You don’t want your potential customers to see this warning when visiting your site.
The good news is that renewing your SSL/TLS certificates is easy. All you need is a little advance planning. Having the certificate renewed can take a couple of weeks, depending on the level of certificate you apply for (refer to Chapter 5 in 'Website Threats for Dummies, A Wiley Brand' for the different levels), so don’t leave applying to renew your certificate to the last minute.