EDIT: I installed Ubuntu 16.04 and updated to 17.10; Unity worked until update. Wayland takes place and is running well until now, except for what I forgot to mention, as follows.
Some official info tells Xorg will be the default graphic server for 18.04; Wayland stays as an second option. As I've experienced some GNOME Shell crashes, I realize the reason 🙂
Found this and executed its steps. At Login Screen, the cogwheel menu (⚙️) shows three options. First of them is "Ubuntu" for Wayland, which works. There are two other options: "Ubuntu on Xorg" don't work, and "Unity", which didn't work anymore.
I have executed @sam-thomas' answer, but Wayland didn't start. So I switched back to get Wayland work. That's why I think this is not a duplicate.
Got some app problems; Gparted didn't start, for example. I needed to use it and nothing appears on screen after password prompt, I could reproduce the case, for which the xhost +SI:localuser:root command gave temp solution, although I know it is not recommended.
I checked this, but as my laptop is a Dell Inspiron 5548 with a AMD Radeon and Intel video cards, that shouldn't fit my case.
That said, what should I do to get Xorg working again?
Best Answer
I realized, after some search and time, that Xorg wasn't even installed on my laptop, possibly after upgrading to 17.10, when Wayland took place over after that. That said, I managed to install Xorg using
sudo apt-get install xserver-xorg --install-suggest. Thus, both "Ubuntu on Xorg" and "Unity" options got to work, although Unity gets slow for some reason not yet identified.