Ok, do you remember choosing to have the graphical login start after you boot or not?
Even if not, try logging in and then the command
startx.
IF that doesnt work you have a problem with your graphics, and some hardware details could help.
If it works you'll be loading into the default desktop of KDE.
You could then change your settings to give you a graphical login prompt, which by default is Mandrake's own, but I prefer KDM.