Why Is My Monitor Dark?

Hello,
I'm Looking For A Solution That Can Solve My Problem Please.My Monitor Is Appearing Darker Than Usual,Now Anytime I Turn On My PC Everything Works Normally,My Monitor Turns On It Stays On For Like 1 Second Then Back Dark And I Can Still See A Dull Display In the Background I Tried Adjusting My Contrast And Brightness But Nothing Seems To Be Working.My O.S is Windows XP,Monitor NVIDIA GeForce 6150 nForce 340.

Thanks in advance for your answer!
Pilot.J