Recover Grub2 using a LiveCD(for Fedora 17)

$ su
$ cd /media
$ mkdir tmp                                                                   # make a temporary directory to mount /boot
$ mount /dev/sda3 tmp                                                 # sda3 is my /boot partition
$ grub2-install --boot-directory=tmp /dev/sda               # install grub2 at the mbr of sda
$ grub2-mkconfig -o tmp/grub2/grub.cfg                       # update grub.cfg
$ reboot