Some systems, hitting any key will do it. Some you hit the sleep button again. Some you hit the power button on the system. Others may have a special "wake up" button... And then, there are systems that can't be woken from sleep due to shoddy motherboards. HP/Compaq systems often fall into this category, so be prepared to pull the plug (literally) if all else fails.
Please help? I know it is very simple but I forgot 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.