Thanks for your help but I solved the problem ...
I did the following:-
1-Logged on to RedHat Rescue system.
2-typed "chroot /mnt/sysimage/ /bin/bash".
3-typed "grub-install z" where z is my hard disk drive,it is hdb so the final string would be "grub-install /dev/hdb".
Thank Godness,Every thing is ok now !!!
Thanks for your help