Sometimes when I command Windows to start hibernation, the screen will go black as usual, but all the fans, including cpu_fan and sys_fans, still stay on. The power LED stays on, and the HDDs keep making their characteristic humming sound. The LED code shown by the motherboard GA-Z87X-UD5H then is 00.
This usually happens if I choose hibernate after the computer has already waken from hibernation. Usually, the computer hibernates correctly the first time after a reboot.
When the computer doesn't hibernate correctly (as described in the first paragraph above) just pressing the power button, at least if I just press it a second and not for multiple seconds at a time, won't do anything. The reset button reboots my computer with a strange DOS screen with just a horizontal cursor as if it's looking for some input, but then after a few seconds it starts the OS normally.
I have already disconnected all external devices except the USB mouse and keyboard, and the DVI-connected monitor, but this doesn't seem to make a difference.