When I start my computer, ther is no display in monitor.Monitor power green light will be lit for 3 - 4 seconds and CHANGE to AMBER light.No message displays over screen. What COULD be possible reasons, whether its VGA is gone or some other reason.

VGA is built in. Power supply is new( I tried with new power supply)No beeps sound. Could you please advise

You need to find out if the problem is with the display or the system. The easiest way to do that is to try the display on a different system (or a different display on your system).My  monitor is Ok , working with other computer.Problem is with the system itself,It not at all booting, even BOIS also not loading
Is the fan spinning? How far does the boot GET?Processor fan , casing & power supply fans r spinning.When I press start button ,power on green light iluminates to green as well as monitor power on light. But monitor grren light will sustain for hardly less than 10 seconds and monitor become off(amber light) while CPU light remains green.There is no message disply on screen.Initially there will be - line at the top of the screen


OS is XP proffes.  2 GB ram ,Power supply is new.Well, I'm no hardware expert but it certainly sounds like the built-in video card is a fault to me.