Well, basically you have to get some kind of boot cd like Knoppix and boot from it. After which you have to mount your root partition like this:
mount /dev/hdax /mnt/your_root
mount -t proc proc /mnt/your_root/proc
After that you have to update your grup config file. I have no uidea how to do this as I use lilo for which i just have to type 'lilo'.
Then just type 'exit' and you can restart and you have your menu back
Hope this helped a bit