Try ctrl+alt+backspace to see if you can hoop back to the kernel and edit its X-Windows configuration file /etc/X11/xorg.conf.
If not you can always boot any Live CD up, make a temp. directory, mount Mandrake partition on on it and edit its X-Windows configuration file.
Your problem is most likely due a incorrect selection of the video driver and that can be rectified simply by changing the /etc/X11/xorg.conf (may be called XF86Config-4) in the
with the video specified something like
Just change nv to vesa or something that fit your video card.
The xorg.conf can be altered in Bash sheel by typing xorgconfig or xf86config too. The full range of video drivers is usually stored in /usr/X11R6/lib/modules/drivers.