BleepingComputer.com: black screen, must reboot

Sometimes, when I sit down in front of my computer, with everything still on from the last session, the monitor will not go on. The little light on the bottom frame of the monitor will not go from an amber light to the green light. And the screen itself stays black. If things are working, all I have to do is move the mouse or hit the spacebar key. I have to reboot if it doesn't work. If you have a solution, please reply.