I don't know how this happened but the Ubuntu logo has been replaced by this one on the picture. I don't recognise this logo (I haven't looked it up).
How can logo of my distribution change all of the sudden ? Can
it be changed back ?

I don't know how this happened but the Ubuntu logo has been replaced by this one on the picture. I don't recognise this logo (I haven't looked it up).
How can logo of my distribution change all of the sudden ? Can
it be changed back ?

That looks like the Ubuntu Gnome logo to me:
What does the new logo of Ubuntu GNOME mean?
Maybe you installed Ubuntu Gnome on the side? Not sure how to put it back, but removing the packages may help.