Never fear, I just had that problem and was able to fix it!
I got a black screen with "GRUB" printed in the upper-left of the screen; no response for the keyboard or anything else. Apparently what happened (and why I don't know) is that the mapping from the way GRUB specifies drives and partitions to the way Linux specifies drives and partitions got screwed up.
There is a file in /boot/grub called devices.map
which you can edit this mapping with.
With mine, this is how the screwed up devices.map looked like
So I changed it to
hd0 in GRUB is supposed to be the same as hda in Linux. Why you'd change that or better yet, why it would change at all, I have no idea. But that fixed it for me. GRUB was able to find the config files and kernels and all that good stuff.
Maybe show me what you're looks like?
-- the dudeaMAn
PS you'll need KNOPPIX or something like that which boots from CD-ROM to access your filesystem of course