Did you even bother to create a bootable disk during the install process when it asked you too? If you did, use it to boot up Fedora Core, log in as root and run
If not, then boot with the first CD and use the rescue mode. Tell it to reinstall GRUB on MBR again or you can use the upgrade option if you wish. When you get to the bootloader, tell it to go ahead and install GRUB on MBR again (and please create a bootable disk while you are at it...)!
I stole this off a google search!