Can't start ubuntu-desktop after upgrade to 18.04

Try logging in to your default desktop environment from the tty virtual console.

1. Open a text-only virtual console by using the keyboard shortcut Ctrl+Alt+F3.

2. At the login: prompt type your username and press Enter.

3. At the Password: prompt type your user password and press Enter.

4. Now you are logged in to a virtual console, and you can run terminal commands from the console.

Run the following command:

sudo systemctl start graphical.target

If that doesn't work, switch the login display manager from gdm3 to lightdm.

sudo apt install lightdm  
sudo dpkg-reconfigure lightdm   
sudo reboot  

sudo dpkg-reconfigure lightdm will open up a new window allowing you to select lightdm as the default login display manager. Use the arrow keys to select lightdm and press the Tab key to put the focus on <OK> and press Enter. Then reboot by running this command: sudo reboot

If that doesn't work either see if you can at least switch to text mode (for troubleshooting purposes) with no GUI stuff like the X server running.

sudo systemctl start multi-user.target