In Hibernate mode all the information that is dealing in RAM is saved to the hard drive and the computer turns off completely. When turning on the PC again, all applications are restored to the previous state before hibernation.
You might want to turn Hibernation Off may be because of low disk space of something else, here is how this can be done :
Go to Start -> All Programs -> Accessories-> right click Command Prompt and then select Run as administrator.
This will display the command window. Now inside command window or DOS window just type :
powercfg.exe /hibernate off
and press enter. This command will disable hibernation on your system. To re enable hibernation you can use command: