Windows 2000 Hibernation Problem

I currently have Windows 2000 Professional and after installing the hibernation tab appeared in the power options. I enabled it and the computer allowed me to enter hibernation and worked fine. But the next time the computer came out of hibernation,the option to go into hibernation dissapered. All I can do it shut down, log off or restart. How do I get the hibernation option to come back. I have latest BIOS updates and drivers on my system. The motherboard is ACPI compliant etc.