kernel /vmroot=LABEL=/ rhgb quiet
looks odd. Generally the kernel image name for RHEL looks something like vmlinuz-2.6.18-<Red Hat specific version>, which in your case would be /vmlinuz-2.6.18-128.el5. Also, there should be a space between the kernel image file and the "root=" option.
Did you copy the output from GRUB verbatim? If so, it looks like your GRUB configuration got borked. You will need to reboot off of a live CD in rescue mode and fix /boot/grub/menu.lst so that it points to the correct kernel image.
If you did not copy the GRUB error message verbatim, it's possible that there's some other problem (e.g. your kernel image was deleted). Can you boot off of the rescue disk. mount your filesystems, and post the results of:
ls -l /mnt/sysimage/boot