I started having some problems on my Windows Vista machine lately regarding sleep/hibernation: Sometimes (but very often) the machine would not wake up. Well, actually it seemed it had woken up correctly and the keyboard seemed to work, but the screen would stay black no matter what I'd do.
I also noticed that when this happened, trying to shut down the machine again wouldn't work either, so I had to resort to completely shut down power to it and reboot from scratch. Looking at the event log, though, it would indeed appear the machine partially woke up, even to the point that the network cards would pick up an IP address. No errors at all on the event log.
However, I suspected this was the fault of a recent hardware change: I had recently gotten a Microsoft LifeCam VX-7000 to replace my older VX-6000 and I had installed its drivers and software (which I'm sorry to say is still crap).
Next time the machine hung while trying to come out of hibernation, I tried disconnecting the LifeCam from the USB port before turning the machine on. No dice; it still hung up.
But then I noticed that if I disconnected the LifeCam before putting it to sleep/hibernate, it would wake up without a hitch! Weird, to say the least. I still don't know what the LifeCam drivers do or why they might cause the issue, but at least I know how to work around it for the time being.