Yesterday I found my self in a situation which I've never seen before on a Linux system: The MBR (Master Boot Record) got wiped. I'm still clueless as to the cause. I had just upgraded to a new kernel through yum, and after a boot where I attached a new SATA drive, a blank screen appeared after BIOS, accompanied my an endless bip. Neither changes seems a likely cause, so I still don't now what happened.
When I started to fix it, I found that I had never needed to reinstall the MBR in the last 10 years of Linux, so I was a bit at loss. Luckily, I had the Fedora install CD, which has rescue mode. It gives you a shell, and from there I was able to find this excellent article.
And the command to restore the MBR? Very simple:
(This is assuming you already did
chroot /mnt/sysimage in the rescue shell, and that you have a SATA disk at slot 1.)