I take it that you have not installed your grub to the MBR on /dev/sda
Common mistake is to install it to /dev/sdaX (X being the partition number)
You can reinstall it from your Ubuntu Live CD.
Once booted into the Live CD session:
sudo mount /dev/sda2 /mnt
then bind the directories that grub needs access to to detect other operating systems:
sudo mount --bind /dev /mnt/dev && sudo mount --bind /dev/pts /mnt/dev/pts && sudo mount --bind /proc /mnt/proc && sudo mount --bind /sys /mnt/sys
Then change root:
Now install grub:
grub-install --recheck /dev/sda
Now exit the chrooted system and unmount everything:
exit && sudo umount /mnt/dev && sudo umount /mnt/dev/pts && sudo umount /mnt/proc && sudo umount /mnt/sys && sudo umount /mnt
Now shut down and turn your computer back on and you should get the default grub screen.