How to undo a command

[.Kencs]

I been having issues trying to open up the Login Screen Settings. Nothing happens after I click Validate. I was told to open Terminal and put this in:

sudo dpkg-reconfigure gdm

I then restarted, and now all I have is 'Automatic Login' and it doesnt give me my account or my wife's account, just logs in automatically. I can't change how it logs in as 1) I cant access login screen settings, and 2) as you can tell with past posts, I know nothing about Linux still.

I'm using Ubuntu 9.10

Please help...I need to undo the changes so I have the available login accounts at the beginning...

thanks