Problem - There is a problem with this website's security certificate. The security certificate presented by this website has expired or is not yet valid. Certificate Error: Navigation Blocked
If numerous, disparate sites are displaying this error, the Date Time on the computer is likely incorrect. In Windows, click the System Tray's Time (lower-right corner) and confirm the date.
If the date was way off, the hardware's BIOS date and time is wrong. Correcting the time above will reset it, but if it continues to fail, the motherboard's BIOS battery may be dying and may need to be replaced. This is likely an issue only on older computers.
Date Time is Correct - Problem only with one site:
If the Certificate Error only occurs on a single site, consider the following:
1. In Internet Explorer (or other browsers), click "Continue to this website (not recommended)"
2. Click the Certificate Error to open the information window.
3. Click View the Certificate.
Examine the cert to see if it is reasonable -- does it belong to the site you are attempting to view. If yes, click "Verify", "Accept" or "Install" the certificate, depending on your browser. It could be a legitimate site has let its SSL certificate expire.
If not, you should be suspicious. If this is an untrustworthy site, or a site you are not familiar with, you should not proceed to the site because something nefarious is going on.