From Fedora Project Wiki
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Note: this is a draft document, and is incomplete.
This document describes how to reinstall your bootloader, the software that starts your operating system. This assumes that you are using the grub bootloader, which is default on Fedora installs.
You may want to reinstall you bootloader if:
- It did not install properly upon install
- Another operating system installed over it
To do so, boot your computer into Fedora rescue mode. chroot to the /mnt/sysimage directory.
The default is for the Fedora installer to put the bootloader on the master boot record. So in most cases running "grub-install hd0" will install it to the MBR.
Exit out and let it reboot.
For more information see the grub manual