Hello.

I've been using KDE for some time, but it got me suprised today. I accidentally pressed some key combination on the left side of the keyboard (I guess ctrl + alt + something or ctrl + shift + something...). The effect was that I got desktop icons on the full desktop instead of them boxed into folder view (as default on KDE), my wallpaper reverted to default and applications dissapeared from task manager (althought they were still running). I have desktop count set to 1 so I guess I couldn't switch to another desktop or anything like that. Any guess what happened? What did I press?

My guess is that you have changed the activity you are currently in.
This would certainly explain the change in your desktop applets and the wallpaper, as well as the disappearing applications in the task manager.

To fix this, click the cashew (top right corner of screen) > Activities. You will probably see another activity called "Desktop" or similar on the left most side of the activity manager (which will popup from the panel). Selecting this should restore your desktop back to normal.

Looks like you switched to a different activity. The default shortcut for switching activities is Meta+(Shift+)Tab (System Settings -> Shortcuts and Gestures -> Global Keyboard Shortcuts -> select Plasma Desktop Shell from the drop-down -> Next Activity and Previous Activity.
Activities are a different way to group different tasks. You can also switch between them by using the activity manager, available by right-clicking on the desktop -> Activities. This will bring up a bar above the panel with icons for the different activities which you can use to switch between them, create new ones, delete unused ones, and do some basic customization.
If you want to learn more, there are a few blog articles around that you can find by searching "KDE Activity". If you don't care for them, you can delete all but the default activity and ignore them.

You're right, thank you very much. As for now I disabled keyboard shortcuts for activities. But I learned something new and maybe I'll find activities useful some day.