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could you please tell me last time when did you get this problem?
This can Happen to the Graphic Card or Screen, press the power switch till it turns off your computer, it could take several seconds so hold the switch down till it switches off, then wait for a minute and switch it back on if the problem persists.
IF EVERYTHING FAILS REMOVE THE bATTERY AS WELL TO TURN OFF
or please try to install vga driver from your driver cd.