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 Microsoft Windows. The size of this file is approximately equal to the amount of random access memory (RAM) that 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.

Hibernation is made unavailable when you set the value of the Hibernate after setting to Never by using the Power Options item in Control Panel. To make hibernation unavailable, follow these steps:

1. Click Start, and then type cmd in the Start Search box.
2. In the search results list, right-click Command Prompt, and then click Run as Administrator.
3. When you are prompted by User Account Control, click Continue.
4. At the command prompt, type powercfg.exe /hibernate off, and then press ENTER.
5. Type exit, and then press ENTER.

To make hibernation available, follow these steps:

1. Click Start, and then type cmd in the Start Search box.
2. In the search results list, right-click Command Prompt, and then click Run as Administrator.
3. When you are prompted by User Account Control, click Continue.
4. At the command prompt, type powercfg.exe /hibernate on, and then press ENTER.
5. Type exit, and then press ENTER to close the Command Prompt window.

Ref: http://support.microsoft.com/kb/920730

(Archived here, so I can find it next time I need it)