Your PC Won’t boot again?
You see error on Grub?
No fear, please. Here what you have to do: boot with a live distro (eg: knoppix) issue a root shell (or su root) and
mount -t auto -o rw /dev/hda2 /mnt/hda2
chroot /mnt/hda2 /bin/bash
where hda2 is, for me, the partition were ubuntu is installed