You will need to create a custom configuration file to accomplish this goal.
Here's how you do it:
Open Terminal (if it's not already open)
Create a new directory for some LightDM configurations (if it doesn't already exist):
sudo mkdir /etc/lightdm/lightdm.conf.d
Create a custom configuration file:
sudo vi /etc/lightdm/lightdm.conf.d/50-custom.conf
This is caused by graphics driver update.
In my case, nvidia-340(put your graphics driver name here) driver patch update case this problem.
Simple solution that I follow is,
First of all, login in your tty shell by Ctl+Alt+F1
And run following commands,
remove graphics driver
sudo apt remove nvidia-340
sudo apt-get install --...