Your distribution should come with a recovery cd which allows you to do that, it is a fairly common problem.
A 'manual' way of fixing the problem is to use a boot cd/floppy to access the hard disk and reinstall the boot manager.
[ Very quick: Download and write HAL91 to a floppy, reboot. At the prompt, mount your root partition into /mnt, also mount proc 'mount proc /mnt/proc -t proc', chroot yourself in. 'chroot /mnt /bin/bash --login', then re install lilo 'lilo', or if you used grub 'grub-install /dev/hda1' [or whatever your boot partition is. Logout of the chroot, unmount /mnt/proc, then /mnt, run 'sync' and type 'halt'.]