The Hiberfil.sys hidden system file is located in the root folder of the drive where the operating system is installed. The Windows Kernel Power Manager reserves this file when you install Windows. The size of this file is approximately equal to how much random access memory (RAM) is installed on the computer.
The computer uses the Hiberfil.sys file to store a copy of the system memory on the hard disk when the hybrid sleep setting is turned on. If this file is not present, the computer cannot hibernate.
To have Microsoft disable and enable hibernation for you, visit these KB920730 links: