After Ctrl+Alt+F3 during login, Ctrl+Alt+F7 only blackscreens
Solution 1:
CTRL+ALT+F1 did the trick, returned to Plasma login screen correctly.
Solution 2:
Assignment of the virtual consoles has changed since Ubuntu 17.10. Virtual console 1 is assigned to GDM. Virtual console 2 will contain your running desktop if you were logged in. Virtual console 3 and higher are terminal consoles.
In previous versions, all graphical sessions were on Virtual console 7.