There are already many topics about this problem. Just search for a thread using the search feature or browse the general category and you'll surely find one.
By summary, you just need to reinstall your boot loader, e.g. grub. To do this, you can boot from a livecd then on the prompt, do
Code:
mount /dev/hdaX /mnt/anydir; chroot /mnt/anydir /bin/bash
then re-install grub from there. If you have no idea how to install grub, 'info grub' will do.
Booting problem
