Sleep mode is an energy-saving mode in which your computer monitor--and sometimes the computer itself--decreases functionality to save energy. The monitor itself appears black. Usually you get out of sleep mode by just pressing a key on the keyboard or moving your mouse around. For more serious cases, where some sort of bug is involved, further steps are needed.
Press a key on the keyboard or move the mouse. If the computer doesn't come out of sleep mode, go to Step 2.
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Restore the hibernate file. To do so, go to Start > All Programs > Accessories. Then right-click "Command Prompt," click "Run as administrator" and type in "powercfg -h on." Press "Enter."
Reset the amount of time that it takes the screen to go into hibernation. On laptops, go to Start > Control Panel > Mobile PC > Power Options. Click "Change plan settings," then "Change Advanced Settings." Select "Allow hybrid sleep on (on battery)." Set "Hibernate on battery" to a time interval longer than you currently have the sleep mode set to, but shorter than the life of your battery. Somewhere between 10 and 60 minutes should work--a shorter time is recommended for safety; a longer time is recommended if hibernate kicks in too frequently for your convenience.
Update the display driver. Go to the download section of Microsoft.com to find the latest for your specific operating system.
Change the settings so the computer doesn't go into sleep mode in the first place. This is a last resort, since it means that even closing the lid of the laptop, it's still using up the battery. On laptops, which seem to have the most sleep mode troubles, go to Control Panel > Mobile PC > Change Battery Settings. Click on your power plan and choose "never" for the setting "Put your computer to sleep." Your laptop will not go to sleep, even when you close the lid.