You might not be able to put your mobile PC to sleep for one or more of the following reasons:
The video card on your mobile PC doesn't support sleep. Update the driver for your video card, or check the information that came with your mobile PC about your video card and supported drivers. For more information, see Update drivers.
Some settings are managed by your system administrator. For more information, see Why can't I change some settings?
Sleep and other power-saving states are turned off in your mobile PC's basic input/output system (BIOS). To turn sleep on, restart your mobile PC, and then enter the BIOS setup. As your mobile PC starts, instructions typically appear on the screen that indicate which key or keyboard shortcut that you must press to enter the BIOS setup process. Because not all computer manufacturers use the same BIOS, different keys are assigned for this purpose. To learn more, check the information that came with your mobile PC or go to the manufacturer’s website.