Usually after an update I also preform this action. It cleans up and mostly removes (the oldest drivers) and leftovers. So there is no harm done ...in my case.
But if you haven't restarted I would try testdisk to see if you can recover those lost folders/files which are important to you. If a recovery is not possible then I would recommend a fresh installation (unless you have a backup) to avoid headaches and all kinds tryouts.
I suspect it was the purge that may have removed all of the configuration files but it shouldn’t have. I’d have to look at the Opera package to see what their config looks like. Sorry you had to deal with that mishap.
Same. I cannot stand Opera.
I do not see how purge or autoremove could remove the Home Directory. I cannot help but think something else happened there...
I don't think there is a need for a fresh installation or restoring a backup (which I have). As I wrote to Aravisian, I recovered all files from external HDD.
As far as I can see, no system files or settings suffered any harm from this "unexpected cleaning". The system runs smooth and I cannot notice unusual behaviour. Hope there won't be any more unpleasant surprises.
Might be a good time to install Timeshift:
When running removal or autoremove commands... always and without exception, Read what the terminal says will be removed. Always.
Autoremove can be problematic if there are PPAs or 3rd party repositories enabled as well. When 3rd party apps have shared dependencies with native ones removing one can affect the other. Having installed and removed the Opera .deb many times I suspect the cause lies elsewhere.
Have it, but I think no need to use it now.
Yes, you are quite right. Though, I'm not sure I wouldn't be able to understand what it said.
Forgot to write 
. Between purge and autoremove commands I type this one, I found in some tutorial on YT: "rm -rf /home/djurica/ .cache/opera". But system refused to execute it, I cannot remember now what was the explanation. Is this the missing point that might explain mess I made? But, I repeat, that command hasn't been executed.
Yes, that was it.
You entered a command to remove the Home Directory.
Even if it hasn't been executed.
It was executed - you know this because your Home Directory was removed.
It may be that the rm -rf command was executed, then the system put up a warning in regards to the tail: ".cache/opera"
As that message is unknown, I cannot verify it.
But auto remove should not remove the Home Directory. "rm-rf 'Home Directory'" WILL remove it - and you input that command - and it was removed. It's an open and shut case.
That being said... there is a TYPO in your command. You have a space that should not be there.
The instructions wanted you to run:
rm -rf /home/$USER/.cache/opera
But you ran:
rm -rf /home/$USER/ .cache/opera
Please see This Thread:
I wish tutorials did not blithely throw the rm -rf command around without giving a crash course on how to use it and how to avoid problems with it.
That YouTube guide belongs in the Wall of Shame.
Okay, I understand. Thank you very much. No more playing with "rm -rf".
This is the tut in question (How to Completely Remove or Uninstall Opera Browser in Ubuntu 18.04 LTS? - YouTube), and as far as I can see, there was a space between / and .cache, both in txt doc and in Terminal. I just want to say that I was very careful in following the instruction.
Yes, it looks like a space between to me, too.
But looking in the "read More" of the description, there is no space visible, there. I think it has to do with how it appears on the video screen.
You clearly followed that very carefully, based on what you could say. And very good that you have backups.
But in future, remember that any command that follows any direct path must have a Full path in order to complete its run of that path.
In the case of a 'rm -rf', this can get dangerous; As white space left behind can halt the path too soon.
In this case, the path halted at your Home Directory instead of at the ~/.cache/opera directory.
You're right. I didn't "read more", just made myself focused on the video.
Okay, I'll bear it in mind. Thanks again, I really appreciated your willingness to help me. And, of course, many thanks to others who jumped in this topic.
Well, when I second thought, I myself made a mess not only of my machine, but this topic too. Sorry, I suppose this is what the first steps of an inexperienced Linux user look like.
Or of a Linux user. 
We all make messes of our machine and of topics.
@swarfendor437 can attest to the multiple times I nuclear bombed my machine early on, trying out all sorts of crazy things.
Sometimes crashing a machine is how we learn things. If it is not a production machine i.e., used for day to day work, then it does not matter - it is all part of the fun of learning GNU/Linux. At least you can do things with GNU/Linux - your hands are not tied to proprietary licences that Microsoft and Apple use - you own the Operating System it is yours and it is what Community is all about! 
That scares me.
