Ubuntu 18.04 Gnome hangs on login screen

Same issue. Even though Ubuntu was released last week, you can not use 18.04 on Dell XPS directly. I installed in several ways but each time laptop freezes as soon as I enter password on login prompt.

I got it working by pressig Ctrl-Alt-F2 at the login prompt and then:

sudo add-apt-repository ppa:graphics-drivers/ppa
sudo apt-get update
sudo apt-get install nvidia-390

And restart the laptop

I had same problem on a Dell XPS. After several installations and downloaded the Nvidia drivers the issue continued, so I disabled the Secure boot option in the BIOS and that worked for me I could pass the login screen. Everything seems fine although I don't have the gear icon in the login screen anymore, I had the Ubuntu and Ubuntu in Wayland options before disabling the secure boot. I just need to fix the grub for the dual boot with the efi partition.

What worked for me

sudo ubuntu-drivers autoinstall