...moving the mouse or pressing the "space bar" or "Enter" key? If there is a "sleep mode" button on the keyboard, did you try pressing it again. Maybe even pressing the power button to shut it down, then restart the machine?
Hope this helps.
Please help? I know it is very simple but I have forgotten how. I have Windows XP and a stupid Compaq keyboard with a sleep button that my daughter accidentally hit. I cannot figure out how to wake the computer up.