Know your Ubuntu. Troubleshooting Guide.

The Bootloader

If your system still fails to start, it is likely that your bootloader is broken. When you use a bootloader such as GRUB, a tiny file (called the boot sector) is copied into the very first part of your hard disk. If this file is broken or corrupted, your system often won't boot. Don't worry though, you can fix this! Restart your computer, and choose the newest rescue mode option. When using rescue mode, the system starts in a limited single-user command line mode. You can use this mode to fix any problems from the command line. In this case, you need to fix the GRUB bootloader.

To do this, go to /boot/grub, and load menu.lst in a text editor. Unfortunately we don't have the space to cover GRUB configuration here, so it is recommended that you consult the excellent documentation on the GRUB Web site at www.gnu.org/software/grub/.