GRUB error is one of the most common error that a GNU/Linux newbie often encounters.
- No Operating System found
- GNU/Linux disappears after repairing Windows
- simply the GRUB prompt
are some of the common errors that occur.
grub-install /dev/sda (from the rescue environment) often solves the problem.
Else try this,
Type the following commands after entering the rescue mode from the OS CD/DVD
find /boot/grub/grub.conf (or sometimes find /boot/grub/menu.lst)
root(hdX,Y) (where X and Y have to be substituted by the result you get after the previous command)