How to repare GRUB2 in two words
Install GRUB1 🙂
I just saw that someone searched for this. As of October 2007 GRUB2 is not perfectly stable yet as well as there are no useful documentation. So, try to make GRUB  rescue disk (or USB flash), boot it (read GRUB manual) and install GRUB1 when the system boots.
If you see that this document is outdated (for example, old more then six months), try to use my grub category for finding newer documentation or try to find some other documentation, including checking GRUB2 home page.