Ubuntu login screen changed to KDE

view story

http://askubuntu.com – I use Ubuntu, With different Desktop enviroments. I had KDE for a while and after a week, suddenly my login screen changed to the look of KDE. There is only one problem: in the Ubuntu login screen, your account name already is selected, but in KDE login screen it doesn't show up, you need to write it all yourself. Now my username doesn't work, so I can't login into Ubuntu and I don't want to re-install. What can I do? I use Ubuntu 12.10 with dual boot Windows 8. Desktop enviroments: KDE, Unity, Cinnamon, Gnome. (HowTos)