How to Restore GRUB

1. Boot from CD and mount the system. We used the install disc because I never bothered to burn a rescue CD, har har. When you get to the splash screen, type "linux rescue" and hit return. It will ask you a couple things about your language and keyboard, and will offer to bring up the network interfaces -- I said no. Then it gave me the option to let it try to detect and mount the system read-writable, or read-only, or no auto-detect. I chose the first option and it worked.
2. Fiddle with grub.conf, if necessary. Open /mnt/sysimage/boot/grub/grub.conf in an editor and check to make sure it looks okay. If you want to revert to a previous kernel option, you can add its parameters here, or they may already be in the list -- newest kernel info gets appended to the top. You can also change the default option (what gets highlighted in the grub menu) by changing the "default=0" line to something else; the first block is 0, second is 1, etc.

   If your grub.conf is missing or corrupted, you're kind of on your own. Here's a sample:

   default=0
   timeout=5
   splashimage=(hd0,1)/boot/grub/splash.xpm.gz
   
   title Red Hat Linux (2.4.18-14)
         root (hd0,1)
         kernel /boot/vmlinuz-2.4.18-14 ro root=LABEL=/ hdc=ide-scsi
         initrd /boot/initrd-2.4.18-14.img
   title Red Hat Linux (2.4.18-openmosix3)
         root (hd0,1)
         kernel /boot/vmlinuz-2.4.18-openmosix3 ro root=/dev/hda2 hdc=ide-scsi
         initrd /boot/initrd-2.4.18-openmosix3.img
   title WindowsME
         rootnoverify (hd0,0)
         chainloader +1
   

3. Write to the MBR. Assuming grub.conf is intact, continue with the process. Type "grub" (return) to enter the grub shell.

   Tell grub where to find the requisite files. If you know where they are, enter something like:

   root (hd0,1)
   

   (hd0,1) means primary controller master, second partition. If you DON'T know where they are, type:

   find /boot/grub/stage1
   

   and then enter the root command with the correct parameters. Note: If you are using a separate /boot partition , as the official documentation says: "... if you have the partition /boot and you install GRUB images into the directory /boot/grub, GRUB recognizes that the images lies under the directory /grub but not /boot/grub.' Thus, if 'find /boot/grub/stage1' does not find the file, try 'find /grub/stage1'."

   Next, type:

   setup (hd0)
   

   This command will install grub on the MBR of the first drive.
4. Test. Type quit and reboot. Everything should proceed normally.

by http://www.whoopis.com/