Sometimes, if you modify the MBR of a x86 disk you can loose its content so you need to reinstall the grub again, for example after install a mixed linux/solarisx86 environment, or migrating a x86 physical machine to a virtualized environment.
You have two options to boot the system, both using the CD-Rom:
One is using the cdrom grub, to do that, press e after grub appears, and delete all the lines and add the next 3 lines:
root (hd0,2,a)
kernel /platform/i86pc/multiboot
module /platform/i86pc/boot_archive
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