Section 4.2.3 of http://www.faqs.org/docs/linux_intro/sect_04_02.html The links take you to the FAQs and more so you can fix your grub.
I have RedHat Linux and Win XP on my machine and was using GRUB to boot either of them. It was working fine. I wanted to move some extra space from Linux partition to windows. So I used Partition Magic to free some space on Linux partition and merged it with Windows partition.
Now when I boot, I am taken to the GRUB command prompt. I tried to boot using Linux Intallation CD and also Win XP installation CD, but I always get to that grub command prompt. I don't know what to do from there. Please help me.