Is the bootloader installed to the MBR? If it is and if the MBR is hosed (it happens--I hosed one yesterday), your best option might be to format the MBR, repartition and format the hard drive (necessary after formatting the MBR in that circumstance), and reinstall from scratch.
If so, first boot to a live CD (Knoppix is great for this) and copy your important data, including any customized configuration files, such as smb.conf and rc.firewall, to a safe place.
If the bootloader is installed to Superblock or to a boot partition, I'm not knowledgeable enough to help you. If the configuration files are accessible with a Live CD boot, you may be able to edit them and fix the problem in a less drastic way.
How to format the MBR under Linux
if you don't have a DOS boot disk available. (Note: In the example, the hard drive is hda; if yours is sda, edit the example accordingly.) With a DOS boot, the command is fdisk /mbr
Please help others help you by posting the exact wording of any error message you receive or a picture of the screen where the failure occurs.