Put CD1 in the drive and boot. If you don't see options to a)boot from hard drive, b) boot from cd, or c) repair or rescue, then select the option to install.
It should then detect an existing installation, and ask you which you want to do: a, b, c, or install.
Choose the repair option, to see if it can repair itself. When that's done, remove the cd, and try to reboot.