I totally messed up Grub when trying to install Debian in a separate partition as a dual boot. I tried and tried to fix things, and finally I stumbled upon the following, which at least let me get back to LM. I was at the point of not even seeing the Grub menu - just a flashing cursor. Hope that this helps someone else in the future:
"Moved to a thread of it's own, seeing as this was tacked to the end of a solved thread.
There are a few more steps that oldfred missed. Boot from the Live CD, and once at the desktop, open a terminal and type:
sudo mount /dev/sda5 /mnt
sudo mount -o bind /sys /mnt/sys
sudo mount -o bind /proc /mnt/proc
sudo mount -o bind /dev /mnt/dev
sudo chroot /mnt
After running update-grub you should see a list of the OSs on your system.
I'm continually adding operating systems to my computer. The one I'm using at the moment has XP, Kubuntu 10.10, Ubuntu 10.10, PCLinuxOS 2010 and Ubuntu+Xbmc 10.10. Everytime I install a new OS Grub gets over written, so I use the above commands to install grub the way I want."
I still can't get Debian to dual boot, but I also think that there is something wrong with that partition now. I think that I'm going to have to reinstall it. Anyway, hope that helps.