Updated to 18.04, PC freezes before the login screen (Ubuntu logo)

My PC freezes before the login screen (Ubuntu logo); if I select RECOVERY MODE and resume boot everything goes well. I suspect a problem with the graphics driver.

Finally I was able to solve the issue by uncommenting the line #WaylandEnable=false in /etc/gdm3/custom.conf

Perhaps i can supplement the answer to @Vishesh Gautam and @Marcelo4k:

In the ArchWiki page on GNOME desktop environment, it is stated that "the default display is Wayland instead of Xorg." Hence, there maybe problem if the default display server is Xorg. It is noted that Ubuntu 18.04 default display server is Xorg. This explains the issue if /etc/gdm3/custom.conf has wayland display server enabled.

@Marcelo4k has successfully troubleshoot the problem by uncommenting the line #WaylandEnable=false, so Wayland display server is disabled and Xorg will be the active display server. This is noted in Fedora documentation as the first step to disable Wayland to run Gnome on Xorg. It further recommended to add a new line under [daemon] section:

DefaultSession=gnome-xorg.desktop

The above recommendation is valid if the result of gdm3 is active (running) using systemctl status:

systemctl status gdm3 

However if gdm3 is disabled and lightdm is enabled perhaps due to upgrade from Ubuntu 16.04 to Ubuntu 18.04. Then lightdm maybe active (running) instead.

So if lightdm is inactive, do as what @Vishesh Gautam has suggested by running sudo start lightdm or running as below to start light display manager:

systemctl start lightdm

However these applies only to Gnome desktop environment. If you have other desktop environments matched with other display managers, you have to consult relevant documentations to your respective desktop environment and display managers. Again, ArchWiki provide a good introduction to Display manager and they give a relevant list on how to match which desktop environment to which display manager.

Hope this helps.