Your going to need a live cd, boot it up and get it to root ( sudo su - )
Remember where you installed your /boot and / to ... like hda or sda
mkdir /mnt/gentoo
mount /dev/hda3 /mnt/gentoo
mount -t proc none /mnt/gentoo/proc
mount /dev/hda1 /mnt/gentoo/boot
chroot /mnt/gentoo /bin/bash
env-update ; source /etc/profile
There are 3 common problems with this error:
1: Bad /boot/grub/grub.conf
2: Bad /etc/fstab
3: Drivers, file system and or hard drive controller
Your pretty much on your own on this.
Check all 3 areas, and use the forums and wiki to help you.
