RHEL not booting. Only shows GRUB _

When you see the following prompt with a blinking cursor instead of your RHEL 5 (Or CentOS/Scientific Linux v5 obviously) server booting, you will have in all likelihood have to re-install GRUB.

 
GRUB _

Step 1: Insert a copy of your RHEL DVD (or mount the ISO if this is a virtual machine) Make sure your machine is set to boot
from the DVD/CDROM drive

Step 2: When you get to the boot screen, type the following and press enter

 
linux rescue

You will have to answer a few basic questions. The last question will ask whether you want to mount as ‘read-only’. Be sure NOT to mount as ‘read-only’

Step 3: When prompt appears, change to real root (‘/’) on your hard disk

 
chroot /mnt/sysimage

Step 4: Run the following command to re-install grub

 
# Replace 'sda' with the device name of the drive you want to boot from. For IDE based disks, this will probably be 'hda', etc.
grub-install /dev/sda

Step 5: Type ‘exit’ twice and reboot the machine. Remember to remove any removable media.
You should now be able to boot into your (non)broken system again.