Grub 2 typically gets overridden when you install Windows or another Operating System. To make Linux control the boot process, you need Reinstall (Repair/Restore) Grub using a Live CD.

Create a live USB and boot system from USB

Mount the partition your broken Linux 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/sda1 /mnt.

# mount /dev/sdXY /mnt

Now bind the directories that grub needs access to to detect other operating systems, like so.

# mount --bind /dev /mnt/dev
# mount --bind /dev/pts /mnt/dev/pts
# mount --bind /proc /mnt/proc
# mount --bind /sys /mnt/sys

Now we jump into that using chroot.

# chroot /mnt

Now install, check, and update grub. This time you only need to add the drive letter (usually a) to replace X, for example: grub-install /dev/sda, grub-install –recheck /dev/sda.

# grub-install /dev/sdX
# grub-install --recheck /dev/sdX

Now grub is back, all that is left is to exit the chrooted system and unmount everything:

# exit
# umount /mnt/sys
# umount /mnt/proc
# umount /mnt/dev/pt
# umount /mnt/dev
# umount /mnt

Shut down and turn your computer back on, and you will be met with the default Grub2 screen.