Sorry, this may be completely irrelevant, but is a hot issue right now:

Are you using a Microsoft site as your home page, such as Bing, or your email mailbox on Outlook.com? If so, please make this temporary configuration change:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste stap and pause while the list is filtered

(3) Double-click the security.ssl.enable_ocsp_stapling preference to switch the value from true to false

I suggest this be only temporary because "stapling" reduces bandwidth use and improves privacy, but some Microsoft users cannot currently access HTTPS addresses on Microsoft servers with that setting on true.

For all other sites... Which error message do you get?

• "Your connection is not secure" -- page generally has an "Advanced" button to get more technical information.
  • What do the security warning codes mean?
  • How to troubleshoot security error codes on secure websites

• "Secure Connection Failed" -- typically has scantier information about the problem and if the padlock appears normally, can be taken to mean "connection failed"
  • Secure connection failed and Firefox did not connect