I suspect, X is not running, try to (re)start it
So you hopefully logged in on the console.
sudo su
I don’t know what DE you have, lightdm or gdm is your display manager?
type:
systemctl status gdm
or try:
systemctl status lightdm
For which you get an answer other “Unit ####.service could not be found.” that is your DM.
Say, it is lightdm.
first try stop it:
systemctl stop lightdm
then start again:
systemctl start lightdm
Do you get an error report about something failed?
If not, press ctrl-alt-f7.
If yes, tell what’s the output of
journalctl -xe.
We may need to examine /var/log/Xorg.0.log