Boot to live USB
Mount the partition we want to restore grub (assume /dev/sda1)
sudo mount /dev/sda1 /mnt
# If /boot folder in another partition (assume /dev/sda2)
# you need to mount it too
# sudo mount /dev/sda2 /mnt/boot
Chroot
sudo mount --bind /proc /mnt/proc
sudo mount --bind /sys /mnt/sys
sudo mount --bind /dev /mnt/dev
sudo chroot /mnt
Reinstall grub
grub2-install --boot-directory=/boot /dev/sda
grub2-mkconfig -o /boot/grub2/grub.cfg
# If EFI
grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg
Reboot