Next time this happens and moving the mouse or typing on the keyboard doesn't work, just hit the power button once. It should come out of Hibernation.
Change this bahaviour in the "Power Options" control panel.
My computer will not power on. Windows XP. The power light is blinking yellow. It appears to be in hibernate mode. Monitor goes to yellow light also. Even if I unplug and replug the power cord, the yellow light still continues to blink. Help