Installing Windows and Ubuntu on the same computer can cause a myriad of problems. GRUB can go crazy and prohibit you from launching Windows or the Windows installation could overwrite GRUB, so you won’t be able to boot into Ubuntu…

The solution is to boot into a live-cd or Pendrive containing Ubuntu, where you have to launch the terminal and enter the following commands:

sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install boot-repair
boot-repair

Just choose “Recommended repair” and follow the on-screen instructions.

Tested this with Windows 10 and Ubuntu 18.10 just a few weeks ago 🙂

Original source:
http://askubuntu.com/questions/240496/how-to-show-grub-after-install-ubuntu-over-windows-8