You can at the Grub prompt test which disk Grub couldn't find.
So far Grub confirm (hd0) exists.
You have two partitions (hd0,0) and (hd0,1). The first has to be your /boot for the Red Hat as the second is type 8e signifying a LVM which no Linux boot loader can read (thats why a separate /boot is necessary). Grub's files, Red Hat kernels and initrd files will be in /boot. Therefore you ask Grub to display the menu.lst (or grub.conf which is symbolic link to menu.lst) by
You then follow each line and type it at the terminal, starting with the "root" statement.
Post the grub.conf and report which line Grub is having a problem.