To re-install the grub2 bootloader in Debian 6.0 , you need to be chrooted into the filesytem, this can be done from the installation DVD or an Ubuntu Live CD. Once you are sitting at a root prompt on the Debian system, you need to type:
Then if you have installed a new kernel from source or changed the configuration of the system bootloader in any other way, you can update the grub.cfg by typing this command:
grub-mkconfig > /boot/grub/grub.cfg
This performs the same operations as running update-grub on Ubuntu. Then you can use ^D to exit the chroot and then type sudo init 6 to re-boot the machine and access your Debian installation!