Depending on which Debian-based distribution we're talking about, you could:
1) Boot from Install CD 1 and use any "rescue" option present to either reinstall GRUB to the MBR, or boot the installed system and then reinstall GRUB to the MBR;
2) If the system is a LiveCD version (Knoppix, Mepis, Morphix, etc), or you have a LiveCD distro available, you could boot that, mount your installed system, use the
chroot command to switch the / filetree to the installed system's filetree, and reinstall GRUB to the MBR.
The GRUB Manual contains all necessary information as to how to (re-)install GRUB.
Hope this helps.