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.