Last Updated on 2014-05-09.
You upgraded manually from OpenElec 3.2.x to 4.0. After the upgrade you are not able to go into standby mode.
The OE / XBMC developers distinguish now between a “real” hard drive (e.g. internal SSD) or using USB sticks or SD cards. On some systems, the sleep mode might be buggy with mobile sticks/cards and is therefore disabled by default.
Unfortunately, this is not mentioned in the GUI anywhere and can be confusing. Under Settings -> Energy Saving, using the Expert view, you see the dropdown button to choose between Shutdown and Hibernate / Sleep, but it is disabled.
You can enable sleep mode again if you add this line to your /storage/.config/autostart.sh: