Most often GNU/Linux becomes hidden after installing/re-installing windows. It is because windows overrides the traditional GRUB from GNU/Linux with
its own boot loader which cannot detect other operating systems. Whereas GRUB is capable of recognizing other operating systems(including Macintosh,
windows etc). So the solution is cut and clear, just re-install the GRUB. But it requires a live GNU/Linux distro. Since I use Ubuntu here a provide a link to download the ISO for Ubuntu because it contains GRUB(don’t think that Ubuntu is the only one with GRUB). If you choose another distro, make
sure that it includes GRUB.
1. Download the live ISO.
2. Use the ISO to make a bootable USB or CD/DVD(better to use unetbootin to make a bootable flash drive).
3. Boot the same before getting into windows by making the first boot device to CD/DVD or any external device in BIOS.
4. Choose the option Try Ubuntu Without Installing.
5. Access the terminal by pressing Ctrl+Alt+T.
6. Find out the GNU/Linux partition by running sudo fdisk -l
7. Mount the partition to some directory
sudo mount /dev/sdaX /mnt
where X denotes your GNU/Linux partition number.
8. Run sudo grub2-install –boot-directory=/mnt/ /dev/sda for GRUB 2 and sudo grub-install –root-directory=/mnt /dev/sda for GRUB.
9. Make sure that the process terminated without any errors.
Reboot the system, access and enjoy the hidden GNU/Linux.