I never had a computer that would wake up from sleep mode. Try pressing the power button. If you still cant wake the computer by pressing power you will have to restart. Either hold the power key for a few seconds until the computer shuts off or press reset.

To disable sleep mode right click on your desktop and click properties. Select the screensaver tab and click advanced or power options (not sure what it's called, we have that tab disabled at the office). From there you can set how your computer behaves after different lengths of inactivity.

Joe