nvidia - no graphical login screen after boot into Ubuntu

I'm using Lubuntu 18.04.3 LTS dual boot with Windows 10.

In the past, there were some problems about display with graphic driver. There was a bug in nouveau that causes ubuntu to crash after suspend. I once tried to disable nouveau (or maybe even remove it) to use nvidia and it completely solved the problem so everything was working fine until last week. After running

sudo apt-get install update
sudo apt-get install upgrade

I noticed that there a new generic header at booting 4.15.0-55-generic along with the old one: 4.15.0-54-generic. However, every time I boot into this new header, I can't get into the graphical login. It only shows a cursor (sometimes blinking, sometimes not) and sometimes I can use Ctrl+Alt+Fx to switch to non-graphical login, sometimes I can't (I have to hold power button to force shutdown in this case).

When select other option, I tried boot into the old header and things are going strange: sometimes I can get into graphical login, sometimes I can't (but I can always use Ctrl+Alt+Fx to login in this case). But now I no longer can get into graphical screen.

What I have tried:

1. follow this answer https://askubuntu.com/a/542490/930755, I see that my greeter is lightdm-gtk-greeter by default (there is only this) and everything were set as he mentioned, except that I have not installed ubuntu-desktop (I think I have nothing to do with this since my laptop was working fine without it).


2. follow this answer https://askubuntu.com/a/162076/930755, nothing changes. There is no cursor, only black screen but I can use Ctrl+Alt+Fx.


3. reinstall nvidia-390. My driver was it, I think I just need to reinstall the driver to solve all the conflicts (in case there are conflicts) but nothing changes.



4. follow this answer https://askubuntu.com/a/532296/930755, after running sudo nvidia-xconfig, it said that xorg.conf has been added, but it also showed a message: package xorg-server not found. And nothing changes.


5. login to non-graphical screen, run sudo service lightdm restart and it shows (spaces are intentionally added as it showed me so):

[ OK ] Created slice User Slice of lightdm
                   Starting User Manager for UID 106
                                  [ OK ] Started Session c1 for user lightdm
                                                   [ OK ] Started User Manager for UID 106 _

I see a cursor at the end. I think this is output in graphical screen (?). Since when I do Ctrl+Alt+Fx, x=1 for instance, I get the previous login (where I type name and password, the messages disappeared)

6. as @Raffa shows, I disabled network connection, run sudo apt purge nvidia-* then sudo apt autoremove then sudo ubuntu-drivers autoinstall then reboot. I see that it install nvidia-430 driver, not nouveau (the default) as expected. And still a black screen with non-blinking cursor, though I can still use Ctrl+Alt+Fx.

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I noticed this: when I do Ctrl+Alt+Fx (for x from 1 to 6) I can switch to 6 different screens and when x = 7, the screen does not switch, cursor disappears (Maybe something prevents it from switching to graphical screen ?)

I would be grateful if someone can help me solve this issue, since the last thing I think would solve it is to reinstall Lubuntu (which I don't want). Thank you in advance.

You need to purge NVIDIA drivers and let the ubuntu-drivers tool install the suitable drivers. To do so, please follow these instructions:

Purge the NVIDIA drivers:

sudo apt purge nvidia-*

Run apt autoremove to complete the purge process:

sudo apt autoremove

Run the ubuntu-drivers tool:

sudo ubuntu-drivers autoinstall

Reboot.

To selectively install the nouveau driver, please refer to this post.