The security certificate presented to this website has expired or is not yet valid.
As mentioned earlier, SSL certificates issued by the Certificate Authorities have an expiry date which is written in the certificate, itself. For example, a standard SSL certificate named RapidSSL Certificate is valid for one year. As soon as the certificate expires, it has no effect remained in it to be able to secure your business and hence is as good as having no SSL certificate installed at all.
Keep updating your SSL certificates before it expires. All the SSL CAs and resellers provides the reminder for the renewal of the SSL certificate before time, check your mails regularly for that. Also keep a personal reminder in your mobile phone or so.