re-install grub using Fedora Live CD. Fedora was installed with LVM. The system dual boots Ubuntu and Fedora.

rescue-grub-with-fedora-live-cd.sh

su
mkdir /mnt/root

# You may use fdisk -l to list the devices
mount /dev/mapper/fedora-root /mnt/root

# Mount the boot partition, maybe /dev/sda1
# As the system has both Ubuntu and Fedora installed, 
# so sda3 in my case
mount /dev/sda<3> /mnt/root/boot

mount -t proc none /mnt/root/proc
mount -o bind /dev /mnt/root/dev 

# change root directory
chroot /mnt/root /bin/bash

# [Re]install grub
grub2-install /dev/sda

# References:
# http://thecreationist.expertscolumn.com/article/recover-grub-using-fedora-live-cd
# https://ask.fedoraproject.org/en/question/40578/how-to-reinstall-or-fix-grub-in-fedora-without-a-recovery-option-on-the-live-cd/?answer=40593#post-id-40593

If you wonder where is your fedora-root you might need to enable first:

$ sudo vgscan
Found volume group "fedora_localhost-live" using metadata type lvm2
$ sudo vgchange -ay fedora_localhost-live
3 logical volume(s) in volume group "fedora_localhost-live" now active

those should be typically root, home and swap, available now on /dev/mapper/*