Autologin

autologin is if a user is automatically logged in after starting the computer with no need to type username and password. When enabling this, you should be aware of some security considerations about physical access. To enable autologin on your computer, find out your distribution and proceed accordingly:

Ubuntu

During the installation of Ubuntu you can choose if you want a user to be logged in automatically. This setting can get lost during a system upgrade. To set it back, you need to find out your desktop environment.

Gnome

Use a text editor to change the Gnome DisPlay Manager configuration file /etc/gdm/gdm.conf. There must be two lines like

AutomaticLoginEnable=true
AutomaticLogin=myuser

If you have this, myuser will be logged in automatically after start. Provided this user exists, so you should change myuser to your user name.