Grub 2 typically gets overridden when you install Windows or another Operating System. To make Ubuntu control the boot process, you need Reinstall (Repair/Restore) Grub using a Ubuntu Live CD.
Mount the partition your Ubuntu Installation is on. If you are not sure which it is, launch GParted (included in the Live CD) and find out. It is usually a EXT4 Partition.
Replace the XY with the drive letter, and partition number, for example: sudo mount /dev/sda6 /mnt
sudo mount /dev/sdXY /mnt
Now bind the directories that grub needs access to to detect other operating systems, like so:
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
Now we jump into that using chroot.
sudo chroot /mnt
Now install, check, and update grub.
This time you only need to add the partition letter (usually a) to replace X, for example: grub-install /dev/sda, grub-install –recheck /dev/sda.
grub-install --recheck /dev/sdX
Now grub is back, all that is left is to 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
Shut down and turn your computer back on, and you will be met with the default Grub2 screen.
You may want to update grub or re-install burg however you like it.