LightDM – LightDM Doesn’t Start: Troubleshooting Guide

Today when I started Ubuntu, it didn't start lightdm, showing a black screen.
I changed to console Ctrl-Alt-F1 and tried to start lightdm manually

sudo start lightdm

It threw me to the graphic console (Ctrl-Alt-F7) and showed the same black screen.
Moreover, it looks like it starts, the console returned the number of process,

lightdm start/running, process 3616

However, after

sudo stop lightdm

I got

stop: unknown instance

Then I started gdm

sudo start gdm

and gdm started normal. So, the problem in lightdm.

To solve the problem I tried to purge lightdm and reinstall it

sudo apt-get purge lightdm
sudo apt-get install lightdm

and

sudo apt-get install --reinstall lightdm

The both ways didn't help.

Now I use gdm, but wants to restore lightdm if it is possible.

Thank you all for you help!

PS: I didn't install recently any additional driver (and haven't NVidia card at all).

PPS: Some additional information

$ sudo start lightdm
lightdm start/running, process 2466
$ dmesg | grep lightdm
[   18.284992] init: lightdm main process (1446) killed by TERM signal
[   57.691273] init: lightdm main process (2466) terminated with status 1

$ sudo start lightdm
lightdm start/running, process 8140
$ sudo cat /var/log/lightdm/lightdm.log 
[+0.00s] DEBUG: Logging to /var/log/lightdm/lightdm.log
[+0.00s] DEBUG: Starting Light Display Manager 1.2.3, UID=0 PID=8140
[+0.00s] DEBUG: Loading configuration from /etc/lightdm/lightdm.conf
[+0.00s] DEBUG: Using D-Bus name org.freedesktop.DisplayManager
[+0.00s] DEBUG: Registered seat module xlocal
[+0.00s] DEBUG: Registered seat module xremote
[+0.00s] DEBUG: Adding default seat
[+0.00s] DEBUG: Starting seat
[+0.00s] DEBUG: Starting new display for automatic login as user stas
[+0.00s] DEBUG: Starting local X display
[+0.00s] DEBUG: Using VT 7
[+0.00s] DEBUG: Activating VT 7
[+0.01s] DEBUG: Logging to /var/log/lightdm/x-0.log
[+0.01s] DEBUG: Writing X server authority to /var/run/lightdm/root/:0
[+0.01s] DEBUG: Launching X Server
[+0.01s] DEBUG: Launching process 8145: /usr/bin/X :0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
[+0.01s] DEBUG: Waiting for ready signal from X server :0
[+0.01s] DEBUG: Acquired bus name org.freedesktop.DisplayManager
[+0.01s] DEBUG: Registering seat with bus path /org/freedesktop/DisplayManager/Seat0
[+0.03s] DEBUG: Process 8145 exited with return value 1
[+0.03s] DEBUG: X server stopped
[+0.03s] DEBUG: Removing X server authority /var/run/lightdm/root/:0
[+0.03s] DEBUG: Releasing VT 7
[+0.03s] DEBUG: Display server stopped
[+0.03s] DEBUG: Stopping display
[+0.03s] DEBUG: Display stopped
[+0.03s] DEBUG: Stopping X local seat, failed to start a display
[+0.03s] DEBUG: Stopping seat
[+0.03s] DEBUG: Seat stopped
[+0.03s] DEBUG: Required seat has stopped
[+0.03s] DEBUG: Stopping display manager
[+0.03s] DEBUG: Display manager stopped
[+0.03s] DEBUG: Stopping daemon
[+0.03s] DEBUG: Exiting with return value 1

$ sudo cat x-0.log
X: cannot stat /etc/X11/X (No such file or directory), aborting.