Will press some buttons and have a look. :D
I am having this very same issue. Booting Windows 8.1 from GRUB causes the computer to simply restart and GRUB to load again. Win 8.1 will boot fine if you change the boot priority in BIOS however I don't want to do this every time I need to boot into Win 8.1. Has anyone solved this problem. I am a newbie to Linux/Ubuntu but I do have a moderate knowledge/skill with computers. Any help would be greatly appreciated...
If you have fastboot still turned on that can be an issue. For whatever reason grub cannot chainload to Windows when hibernated.
WARNING for Windows 8 Dual-Booters
It defaults shutdown to a hybrid hibernation/off state for fast boot
But then files may be corrupted similar to Windows 7 Hibernation:
Fast Startup off/hibernation
Force removal of hiberfil from Ubuntu
User who fixed Windows 8