I have a security certificate installed on my Ubuntu server, running Apache. When I visit my site using the HTTPS protocol, the green certificate appears at the start of the URL. However, when I use the regular HTTP protocol, the certificate does not appear. This used to be working, but I must have changed something to break it and I don't know how to get it working again. Any ideas? Thanks
Er, that's the way it always works. If you are actually looking at a website via HTTP, you're very specifically not using HTTPS, and thus not using the certificate.
You might be thinking of HTTP->HTTPS redirection; you can configure a webserver to redirect all HTTP requests automatically to HTTPS. Even though you type in HTTP, when the page finishes loading, you will see that it is now actually using HTTPS.
So, figure out how to fix HTTPS redirect on your server. You can do this more than one way. A common way is with mod_rewrite, which might help you figure this out. If you were doing it some other way, you'll need to do some more research before we can help you.