You shoukd first try to boot with another videomode, at the first boot screen you can change some options (see at the bottom of the screen), I guess pressing F4 enables you to choose failsafe mode.
Perhaps you can then enter the graphical mode (X windows session) and change back to the original graphic card settings.
Use Display Manager once inside KDE to see which driver is advised for your videocard.
If nothing succeeds you have to manually edit the file /etc/X11/xorg.conf in the terminal. (What I post below is from what I roughly remember, perhaps there are some small errors hereafter)
You could then cd to /etc/X11/
and open the texteditor vim (I guess)
sudo vim xorg.conf
set videodriver back to e.g. vesa, save and try to boot up again.[/i]