This could be because “Allow hybrid sleep” option is set to true. In order to change the setting,
- Open Power Options UI (Control Panel > Hardware and Sound > Power Options)
- Click on “Change plan settings” link against preferred power plan
- Click on Change advanced power settings link in “Edit Plan Settings” screen
- Expand the item “Sleep” and then sub item “Allow hybrid sleep”
- If the Setting is “On” then click on it and change to “Off”
Once this option is set, you can see “Hibernate” option is back in the start menu.