Ubuntu 18.04 fails to start, please see the error message in the picture, the computer will stop to work and only display that message

Question

This is a photo of my screen whenever I push the power button to boot:

Answer 1

Whoever faced this issue, the reason for me was I mistakenly changed /var/lib permission to a different user than root, I have solved the issue by these steps:

1- switch on or restart your machine.

2- before seeing the Ubuntu logo press shift key.

3- Select the line which starts with "Advanced options".

4- Select the line ending with "(recovery mode)". Ubuntu GNU/Linux, with Linux 3.8.0-26-generic (recovery mode).

5- After a moment, your machine should display a menu with a number of options. One of the options will be "Drop to root shell prompt". Press Enter with this option.

6- Then I prompted to enter my root password, after entering my password I went to that specific dir /var/lib which I changed the permission and changed back to the root.