When your laptop suddenly loses its ability to go into Stand By or Hibernate mode, it means that there might be a problem with the power-management hardware or software.

First, check with your computer manufacturer's Web page to see whether you can find any additional information or software updates.

Second, ensure that both modes are activated: Check the Power Options icon in the Control Panel.

Finally, confirm that other hardware or software isn't interfering with the power-management software. If so, remove the interfering software or hardware, or check for updates that don't mess with your laptop's power-management system.