Assuming you have an SSL certificate installed and running in your account. First make sure you are typing the correct domain exactly the one for the one your certificate was issued. Keep in mind is not the same https://www.yourdomain.com than https://yourdomain.com if the certificate was generated for www.yourdomain.com you have to use that exact domain name through https.
Also make sure all your site are over https connection and it do not have any element loading from http.
For example many times clients have images or other elements loading from http so when you load this page through https and you have elements like images or style sheets loading from http the certificate will show an error.
You can correct this either changing the linked element to https (if supported) or if the element is a local element use a relative path instead the URL.
For example if you have a video inserted in your page from http://youtube.com/myvideo.swf then if the external domain support https change it to https://youtube.com/myvideo.swf if the element is a local element and you have it linked with the absolute url like http://mydomain/myimage.jpg change it using the relative path like /image.jpg without including the url section.