users in sudo list are deleted after install kvm?

[Li Li]

hi all,
    I am a newbie of linux(and also kvm). I want to use kvm to create
virtual machine. So I follows this article
http://www.howtoforge.com/virtualization-with-kvm-on-ubuntu-12.04-lts
    I have successfully restarted the network. I can connect to other
machine and visit google.
    Then I reboot it. After that, my account(which is added in sudoer)
and another account(also in sudoer list) are gone! I can now only use
a normal account to login this machine.
     What's wrong with it?