Reverting from Ctrl - Alt - F1
Ctrl+Alt+F1 to F6 are the virtual consoles provided by the getty
/agetty
programs.
Ctrl+Alt+F7 is the console where your X server is running. The GUI (Gnome/KDE or any other) runs over X.
So to get back into your GUI window manager: type:
Ctrl+Alt+F7
or
Alt+F7
or
Ctrl+Alt+F8
With 17.10 and newer, the login screen is on virtual terminal 1, and logged-in users' GUI sessions on VT2 and onwards. So, you may need to use Ctrl+Alt+F2 or Ctrl+Alt+F1 instead.
When you are in a virtual console just press Alt+RightArrow or Alt+LeftArrow to move to next/previous virtual console respectively. Console with X is usually seventh.
To revert to the GUI desktop you have to press:
Ctrl+Alt+F7