Odd one: user complains that when they leave their desk for several hours, when they return they cannot get the computer to “wake up” – but if they turn their computer off, when they return and turn power on – everything comes up fine.
Vista Enterprise x86 – it seems that the power control panel applet, setting hibernate to “after zero minutes” doesn’t truly disable the hibernation feature. In this case, it still attempted to hibernate after a few hours (much longer than the 1 hour to sleep – which did work, and did resume by a simple keystroke).
Instead you need to use the powercfg.exec command to truly disable hibernation (I found this largely by accident: when clicking on the GUI shutdown, log off, restart, sleep I noticed the “hibernate” feature and did a double-take).
Microsoft support article for WS2008, Vista, and Windows7: http://support.microsoft.com/kb/920730