|Which video card did you install ?|
What is the (max total combined output) wattage capacity of the power supply ?
A common problem people have these days is they have installed a video card that requires that your system has a recomended minimum capacity that is a lot higher (more than 50 watts higher) than the existing power supply has.
Either the system will not boot normally at all when the video card is installed,, or the system does boot anyway but the power supply is being constantly overloaded, and is damaged, in a short time, or after a longer time.
If you were using a video card in a slot previously, try installing that again, and connecting your monitor to that.
If you were using onboard video previously, try removing the video card, and connecting your monitor to that.
If the power supply has not been severely damaged, that should allow your system to work.