I’m not sure I agree with that. The default as per the included sample app.yml is to not use the web.ssl or web.letsencrypt.ssl templates and the lack of any HTTP redirection described by @Mads just sounds to me like that default has been kept, i.e. it never had SSL.
That seems pretty promising as being the server hosting Discourse. If you still want to shut it down, you could just power off that Linode but depending on how it was set up, there could be other things on that server as well.
As @sdpiowa said, you can reset the password as described in the link he provided. Scroll right to the bottom to
Resetting the Root Password. You don’t need to do this if you already have the root password for that server though.
With the root password at hand, the simplest way of accessing the server is going to be using the LISH console on the Linode cloud manager.
At the last step when you see
login: in the console, you would type
root and press enter, then type the password and press enter again. At this point you should have a shell and be able to run commands on the server - note that you should be careful not to make mistakes here, most likely a mistake would just lead to an error and nothing happening but mistakes in a root shell can lead to damaging side effects.
Before going any further, be sure to create a manual Discourse backup and after that has completed, take a manual snapshot in Linode cloud manager. This way you know you have everything you need to recover if anything goes wrong.
Start by changing directory to get you into the right place:
Linode images generally have nano installed which is a pretty easy to use text editor. Edit the app.yml file:
You probably can’t use your mouse to move the cursor so use the arrow keys to move around the file and make the changes described previously. Once you are finished with that, press CTRL+X to exit and you will be asked if you want to save, press Y to answer yes and you will be asked for a filename, just press enter and it will save the file and exit nano.
Finally, rebuild Discourse and do not close the LISH window after starting this:
./launcher rebuild app
If all goes well, probably 10-20 minutes later Discourse will be back up and running with SSL.