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The first command you'll want to run is powercfg -a, which dumps out a list of sleep states available to the system.
If some of the sleep states are not available, powercfg will attempt to report why they can't be used.
And sometimes there's not one culprit but a whole collection of elements interfering with sleep.
Since Windows tends to be finicky about being the primary OS installed on a computer, it will not hibernate if it detects that the partition marked for startup on the first hard disk is not itself a bootable Windows drive.
Side note: Hibernation is not always available as an option in the Windows 7 shutdown menu.
The most common reason a system can't hibernate is because the hibernation file -- the file on disk where the contents of memory are written when the system hibernates -- has been deleted or hibernation has been explicitly disabled. This command should also provide feedback if hibernation can't be re-enabled.
Windows can also fail to hibernate if the system was set up to dual or multiboot with another operating system.
This is partly because by default Windows 7 sets up "hybrid sleep" in which the system is suspended but a hibernation file is also written out to disk as a backup in case of a power failure.