I get forgetting the steps so here’s a reminder.
When you dualboot with windows and install windows *after* Linux you’ll lose the grub menu and get into the proper OS anymore.
That’s not ideal.
So here the steps to restore the grub bootloader after windows has removed it.

  • Get a Linux live USB/CD/DVD and start it.
  • Start a terminal and type this
    sudo mount /dev/sda1 /mnt
    
    sudo mount --bind /dev /mnt/dev
    sudo mount --bind /proc /mnt/proc
    sudo mount --bind /sys /mnt/sys 
    
    sudo chroot /mnt
    
    update-grub
    
    sudo grub-install /dev/sda
    
    exit
  • reboot

Done.