GRUB, which stands for the GRand Unified Bootloader is the default boot loader in Linux these days ( it replaced LILO). When your server boots, the system BIOS transfers control to the Master Boot Record of your first boot device which is where Grub is installed. If the removed, damaged, or overwritten, then you will not be able to boot, and in which case you will need to repair/reinstall grub.
The entire process only takes a few minutes if you already have a Redhat/Centos cd to boot of off. Just slap that sucker into the cd drive (or virtual cd drive) and at the Boot Menu type “linux rescue”
Then run grub as shown below.
Next identify your /boot partition
Then install first stage grub into the MBR
# setup (hd0)
All you ever wanted to know about Grub can be found below