Starting from Chrome 66 will ending support for Symantec SSL/TLS certificates issued before June 1st, 2016 and you have to replace that certificates, starting from April 17th onwards this is what your site will look like to your visitors:

To check if your certificate will be affected by this change, you can visit this page and enter your website’s hostname in the form provided: https://www.websecurity.symantec.com/support/ssl-checker

You can find more info on the Official Google Security Blog