If you are referring to when you view a site through your web browser, and your SSL session is terminated, then check to be sure that the URL you are using begins with ‘https’ as opposed to ‘http’. If the page does not show up with a ‘Page Cannot Be Displayed” error, and your URL does begin with ‘https’, then that page is no accessible through the SSl protocol.
If you are referring to your SSL certificate installed on your server, then there could be a number of reasons. Is the firewall allowing traffic through port 443? Did you delete the SSL certificate files? Is there a configuration change on your server software? There are too many possible reasons to list here. Do a refined search on this knowledge base to determine what the cause may be. Searching your server logs for an error message will also help you find the specific solution to your issue.