Sleep mode is a power-saving function where a computer shuts itself down into a hibernative state due to inactivity. While not a full shutdown, sleep mode does allow the computer to turn off the monitor and most of its programming until the user is ready to use the computer again. If your computer monitor is going into sleep mode and you can't get it out, you'll need to check the computer to make sure everything is working properly.
Check the power cable to ensure the monitor is properly plugged in and that the outlet is getting power.
Look at the cables connecting the monitor to the hard drive. A loose cable could result in a loss of power intermittently that forces the monitor into Sleep Mode. If any of the cables are frayed or if the connections are bent and not fitting correctly, they should be replaced.
Hit any key on the keyboard. This should bring the monitor out of sleep mode and back to regular function. Moving the mouse may accomplish the same thing.
Turn the monitor off and then turn it back on. This resets the monitor and could fix the problem.
Reset the entire computer.
Turn off sleep mode entirely by accessing the computer's properties. In Windows XP, right click on your desktop screen and select "Properties." Select the "Screen Saver" tab and then select "Power." Click "Never" next to all the "System Standby" options and then click "Apply." In Vista, access the Control Panel through the Start Menu and click "System and Maintenance" and then "Power Options." Click on "Change Plan Settings" and then turn off all the sleep option settings.
If your monitor is still switching into sleep mode while you are using the computer, it likely needs servicing.