Hardware or software prob donno?

Hello,
I hav recently upgraded my comp with my nvidia grafics ,ram and hdd but the prob now i am facing is i can run those games whodont require grafics but as i run the game which requires grafics as the game starts and the trailor all finishes whn the game is just to start at a moment my monitor turns black and whn i tried the same case by keeping my cpu open i observed tht as the game starts and whn the monitors screen turns black the processors cooling fan stops but the cpu is still on i stopped the power suppy to avoid the processor frm getting burned . While surfing and discussing this prob i found a solution of my monitor refresh rate it was by default high i reduced it it helped a bit but still the prob occured again . Guys plz help me