Does the monitor just not work in X or you can't see anything at all?
If you can see a console then your probablly fine. Boot up linux in rescue mode. If you have a mandrake disk insert it as if you were going to do an install. You should see an option to do a rescue. I have not used mandrake in a 'very' long time but I'm sure the install disk is also a rescue.
Once your partitions are mounted, you know where their mounted, and you have a shell then chroot if mandrake has not already done that for you.
chroot /mnt/yourlinuxpartition /bin/bash
If that is succesful then edit a file with your favorite text editor. I like nano or pico, and there's always vi.
nano -w /etc/inittab
find this line 'id:5:initdefault:'
Change the 5 to a 3
Now your computer should boot up in runlevel 3 and take you to the login promp. Log in and run xf86config or xorgconfig as root and tell it the monitor you have and then try it out. To get your box to boot up into graphical mode again edit the file above and change it back to 5.
That's a pretty narrow attempt to help you out but you didn't include a lot of details so my idea could be totally useless to you but I hope you get it fixed.
