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Windows alongside it, your GRUB bootloader will be overwritten by the Windows bootloader, and your computer will only boot into Windows.

Recovering the GRUB2 Bootloader after a Windows Install  is simple. When finished, your Bootloader should detect all installed Operating Systems and automatically add a bootable menu item for each of them.

Note: This GRUB2 Installation process assumes that a Linux Partition still exists. If you overwrote Linux with Windows, this tutorial will not work for you, as there is no Linux system to recover boot.

How to Restore GRUB2 after installing Windows:

1. Boot from an Ubuntu Live CD or Live USB
2. Once up and running, Open a Terminal 
3. Type sudo su (press enter after typing each command)
4. Type fdisk -l
5. Note which device contains your Linux partition (IE: /dev/sda1)
6. Type mount /dev/sdx# /mnt (replacing x# with your actual device and partition number)
7. Type mount --bind /dev /mnt/dev
8. Type mount --bind /proc /mnt/proc
9. Type cp /etc/resolv.conf /mnt/etc/resolv.conf
10. Type chroot /mnt            
11. Type grub-install --recheck /dev/sdx (replacing x with your actual device)
12. Type reboot (to reboot your PC)

Make sure to remove your Live USB or CD. Upon reboot you should be presented with a GRUB2 menu. However, Windows is missing. Now, I show you how to fix that.

Making GRUB 2 detect Windows Installs:

1. Proceed to boot into your Linux environment.
2. Open a terminal and type sudo update-grub (enter your root password when prompted)

If all went well, grub should have updated it's menu entries to reflect what it detected. Including existing Windows partitions.