Ouch! I've been in that possition before. Here's the easiest way I know to fix that:
1. Download and burn
Knoppix (Download the ISO)
2. Insert the CD and reboot (Knoppix is LIve CD Linux)
Ok, now what you need to do is link the grub config file, which will either be /boot/gub/grub.conf, or /boot/grub/menu. something, to your kernel file, which should be like /boot/kernel-2.6. something.
A good guide for how to do this can be found
here. Now the first thing that you'll realize is that that link is not about Mandriva. Basically, you need to ignore anything that tells you to "emerge" something or use grub-install.
Ok, this is what the Gentoo config file looks like:
# Which listing to boot as default. 0 is the first, 1 the second etc.
default 0
# How many seconds to wait before the default listing is booted.
timeout 30
# Nice, fat splash-image to spice things up
# Comment out if you don't have a graphics card installed
splashimage=(hd0,0)/grub/splash.xpm.gz
title=Gentoo Linux 2.6.11-r3
# Partition where the kernel image (or operating system) is located
root (hd0,0)
kernel /kernel-2.6.11-gentoo-r3 root=/dev/hda3
# The next four lines are only if you dualboot with a Windows system.
# In this case, Windows is hosted on /dev/hda6.
title=Windows XP
rootnoverify (hd0,5)
makeactive
chainloader +1
Go to /boot and ls:
Look for anything starting with kernel. Then go into your grub config file, and put the name of the kernel file in it, for example, if your kernel is mandriva-2.6.11, then in place of :
kernel /kernel-2.6.11-gentoo-r3 root=/dev/hda3
you need:
kernel /mandriva-2.6.11 root=/dev/hda3
Your grub config file should already be written for you by Mandriva installer, but if not let me know and we'll work something up. Good luck!
-Peter