From: https://www.linux.com/learn/how-rescue-non-booting-grub-2-linux

ls
ls (hd0,2)/

grub> set root=(hd0,2)
grub> linux /boot/vmlinuz-3.13.0-29-generic root=/dev/sda1
grub> initrd /boot/initrd.img-3.13.0-29-generic
grub> boot

How do you know the correct partition? 
hd0,1 = /dev/sda1.
hd0,2 = /dev/sda2
hd1,1 = /dev/sdb1. 
hd3,2 = /dev/sdd2