I accidentally restored BIOS on my dell Inspiron 5559 which was dual booting earlier with Windows 10 and Ubuntu-16.04. After restoring BIOS, GRUB got deleted which I repaired using 'boot-repair' . I restarted the system and GRUB successfully loaded. I wanted to make sure that windows was okay or not so I booted in the windows. Problem arose when I restarted from windows. GRUB was nowhere to be seen after restarting from windows. Please help!