I have a Sony VAIO Windows 8 Laptop. At some point, after playing around with various things, it wouldn't go into sleep mode anymore, that is, when I pushed the power button or selected "Sleep" from the Power menu, it would blank the screen, do some disk activity and then sit there with black screen but without doing anything.
I could wake the screen up again by either touching it or pressing any key.
Reverting to previous versions of drivers didn't help. Sometimes it also wouldn't reboot.
What did help, however, was disabling Hybrid Sleep, but I didn't want to do that.