After installation RHEL 6 stop at GRUB prompt

[Liaw Ivan]

Hi All,
 
I'm encountered a problem with install RHEL 6 on my old box ( 2006 ). As subject described, it stucks at GRUB prompt. I had tried root (hd0, 0) and (hdX, X) under GRUB prompt but the reply is Error 21: Selected disk does not exist. And I had tried to install boot loader in first sector of hard drive. But the result is still the same. Could anyone tell me how to fix this problem? BTW, this box has the same problem with Fedora 14 but it works fine with RHEL 5.5.
 
BR,
Ivan