If your dual boot with windows does not show the “Grub boot loader”, then
1) Turn off “Fast Startup” by going into Control Panel -> power settings -> check off “Turn on Fast Startup” option
2) Startup windows. Open Administrator Command window by pressing window+x and choosing command (Admin). Then enter following
bcdedit /set "{bootmgr}" path \EFI\ubuntu\grubx64.efi
.
Restart your machine.