If you install Ubuntu, then Windows in a different partition, the GRUB boot loader vanishes. But don’t panic and reinstall Ubuntu — you can recover it.
Boot to the Ubuntu Live CD, open up a Console window and type:
# Create a directory called “mounted”:
mkdir mounted
# In there, mount the partition where Ubuntu is installed:
sudo mount /dev/sda2 mounted
# (For the above step you need to find the partition name on your own.)
# However, replace the /dev directory in the mounted partition with the
# current /dev directory:
sudo mount –bind /dev mounted/dev
# Change the root for now
sudo chroot mounted
# Finally, install grub to the first SATA disk:
sudo grub-install –recheck /dev/sda
For more information:
https://help.ubuntu.com/community/RecoveringUbuntuAfterInstallingWindows