Ubuntu hasn't shipped with an xorg.conf file for several years now, all of your hardware should be detected automatically so there usually isn't any need for one.
This doesn't mean that you can't use one though, you can create it and its settings will take precedence over any auto-detected settings.
If you want to create a basic xorg.conf you can do the following...
Hit CTRL+ALT+F1 to switch to a tty and log on.
Run the command
to stop the X server, then run
sudo service lightdm stop
to create a new xorg.conf file.
sudo Xorg -configure
Copy it to the correct location with
and you can then restart the X server by doing
sudo cp /root/xorg.conf.new /etc/X11/xorg.conf
sudo service lightdm start