For some reason the top toolbar (the one with File View Image Layer etc has gone missing in my Krita and I can't find a way to bring it back. Is it some bug, or can it be undone with just some unknown shortcut? I've found a post suggesting ctrl+m but that's a shortuc for colour adjustment now...

Sounds like the canvas only mode messed up the menubar for you. Easiest is just to remove your kritarc settings file (Windows: /Users/You/Appdata/Roaming/krita/share/config/kritarc, Linux: .kde/share/config/kritarc)

Ok I found a solution. After opening an empty Krita (without loading any image), you can press ctrl+m to make the menu bar reappear.

This feature is really more like a very unpleasant bug, as you can easily disable half of Krita functionality (functions accessible from the menu bar) by accident and then it's not easy to figure out what's happening. To be honest I don't really see any sense in having this kind of shortcut at all But if it's needed for some reason, it would be great to change the shortcut to something that is not easy to press (ctrl+something is used often for many things) and mos importantly for something that is not set to other function when an image is loaded.

Me neither, though I actually use it in Okular. But for Krita... It's part of KDE's default functionality but with 3.0 we have forked the code that implements it so we can remove the shortcut. We still need to be able to hide the menubar for canvas-only mode, though.

I removed the default shortcut in http://commits.kde.org/krita/3c3799e311 ... bfbf4f6d82

EDIT: Ok, I'm fine after all there is still one shortcut for fullscreen working: ctrl + shift + f

One more problem with this function can occur now. If user manually click in the view menu the fullscreen view, all menus dissappear and there seems to be no way (?) to turn it back (unlike canvas only mode, which can be easily switched on and off). Which I just did to test and now again I don't know how to turn the mnus back on