Boot a liveCD, mount the fedora root partition, chroot into it, fire up the grub command line, select the /boot partition, and install grub. :
#mount /dev/sda1 /mnt/root
#chroot /mnt/root /bin/bash
Then reboot your computer, GRUB should be back. Substitute sda1 by your root partition; root by the mount point of your choice; hd0,0 by the disk/partition numbers (the first disk being 0, same for the first partition.