For complete solution click this link.
Run windows command prompt as Administrator and type:
bcdedit /set {bootmgr} path \EFI\ubuntu\grubx64.efi
Windows 8 removes Grub as default boot manager
For complete solution click this link.
Run windows command prompt as Administrator and type:
bcdedit /set {bootmgr} path \EFI\ubuntu\grubx64.efi