When removing a desktop, such as Kubuntu, you should first install Gnome and
logoff. enter Gnome from the sessions at the bottom of the logon screen.
complete you de-installation of Kubuntu from there. It is the same for any
desktop. Make sure that you are out of the desktop that you are attempting to
remove and that your new one is working first. You can have as many desktops
installed as you want and can switch between them before you login.

The most likely source of your problem is that you removed KDM before GDM was
setup properly. Removing KDM used by Kubuntu would remove your desktop manager,
but it would stay in memory because it is in use. GDM would be installed with
Ubuntu, but you would need to explicitly tell it to use GDM as GNOME would use
KDM which was previously installed and in use in memory. When you logout, KDM
would be shutdown and lost from memory. GDM would not start up because Gnome
would not be configured to use it. If you type startx it will likely tell you
that there is no desktop manager present. That is only one theory, but it can
happen, if you are unfamiliar with having multiple desktops.

Have you considered installing Mint? It is easy to use and it uses Ubuntu's
repos. Assuming the answer is No, then try the ones below.

From the terminal you can try to login by typing: login. You can try to launch X
(the graphical interface) by typing: startx. You can reconfigure X by typing:
sudo dpkg-reconfigure xserver-xorg. It will ask you a bunch of questions, but
the defaults should get you working. I would try those things in order before
re-installing Ubuntu below.

You can use bash commands from the terminal to install missing pieces. Type:
sudo apt-get install ubuntu-desktop or sudo apt-get install gnome.
They will give you a working desktop. It will ask you for your password.

Let us know how you make out.