Almost every distro has its own way to enter "rescue-mode", a text terminal where you have root permissions and can give the badly needed commands to save your install or restore the access to the GUI environment.
There are 2 ways to enter "rescue-mode" in Ubuntu ( Kubuntu, Xubuntu ):
1). The easiest one is to choose:
Ubuntu, kernel 2.6.XX-XX-XXX (recovery mode)
from the boot menu.
If the boot menu is hidden, simply press the Esc key to make it show up.
2). The other way is to use the installation CD, and at the very first screen, at the "boot:" prompt type "rescue" and next follow the instructions that are given.
PS: You can find a lot of helpful Ubuntu info in the Ubuntu Starter Guide