after upgrade 18.04 no gui available only tty

I experienced the same problem and solved it by installing ubuntu desktop using this commandsudo apt-get update then sudo apt-get install ubuntu-desktop N.B Your computer should be connected to the internet, for my case just plugged in an internet cable and used the commands above

Try startx. I had the same problem but it worked for me. The solution is to start lightdm by yourself. The installation is indeed broken in some systems. startx command starts the GUI process for you.

Check status of display manager by executing - sudo systemctl status lightdm

Try restarting display manager by executing - sudo systemctl restart lightdm

If this doesn't work, then you should try reinstalling Ubuntu.