After installing a new operating system on a dual boot machine, or depending what changes we do to our configuration our MBR can became corrupt, the symptom is very clear, the machine won't boot, will simply sit there and display word "GRUB", and stay there :).
What always worked for me is to reinstall GRUB from the rescue environment :
1) Boot the system with a rescue CD
a) We type linux rescue .
2) The system will mount your root file directory under /mnt/sysimage.
3) Change the root directory to /mnt/sysimage
# chroot /mnt/sysimage
4)Get in to the grub prompt by typing grub in the shell
5) Tell grub where to find the boot partition in this case the grub partition was /dev/hda1
**** If you don't know where is your grub partition you can always type
6) To install it on hd0, that is MBR of the first HD.
7) And exit the grub prompt
That is it reboot and you system will beautifully be back .