I've got red crossed https on my gmail's url.
I'm using Windows 7 Professional and the browser is Google Chrome. How can I fix it?
You should click the padlock icon to get more information about why it fails. One reason I've encountered for this problem is that the computer clock is not set correctly. This means that the browser thinks the certificate was issued in the future, and will reject it as invalid. This is common if for example the BIOS battery has gone dry and it resets to some old default date.
Additionally, Windows won't let you synchronize the time if the set date is too far off from reality. (D'oh!) So in case this is your problem, you first need to set the date manually, then synchronize the date to get the exact time.
It can mean several things, often you will get a message to explain. If not, click the padlock sign
In this example case (try it!), the server is using the wrong certificate. Maybe you have clicked a link to a phishing site and some criminals are rubbing their hands with glee. Maybe the site administrators are incompetent and you shouldn't trust them with your private stuff.
In other cases it may mean that the page is vulnerable to cross-site scripting (e.g. some elements are loaded from a non-HTTPS URL)
Here's what I see at https://mail.google.com (it temporarily redirects to a different URL for logging in)