ComputingConfidence

In case you have dual boot and the other boot is MS-Windows, then you could experience the loss of the groub menu/loader, with the result that the machine starts the Bill Gates system directly without giving you a chance to select which one to boot.

By starting on the Ubuntu live CD and opening a terminal window, you can resolve this issue with the commands below.

In a terminal
cd /
sudo -s -H
mount -t ext3 /dev/sda2 /mnt
 
mount -t proc proc /mnt/proc
 
mount -t sysfs sys /mnt/sys
mount -o bind /dev /mnt/dev
chroot /mnt  /bin/bash
 
grub-install /dev/sda
 

If your Ubuntu installation partition is sda2 keep sda2 on the next command or replace it with the right one.

In a terminal
grub-install /dev/sda2
 

Hope it worked for you.  If not, there are great rescue disks you can download and burn. These disks are normally based on some minimal Linux and are fast loading.