Most likely it’s because of a wrong date and time on your computer. Check it by clicking on the clock or date/time icon on your taskbar. Also keep your browser updated; this will update the certificates.
If something like the Heartbleed bug hit the site, they may have revoked the certificates when they fixed it. Flush your cache, reboot, and update your browser.
Finally, make sure you’re really at the site you want to be at. Look at the domain name in the address field.
The Tech Guy says nine times out of ten, it’s a clock error.