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Do you see no video at all, other than possibly a message from the monitor itself? E.g. No signal.
Or - do you see video while booting, but then when Windows is supposed to load, you have no video?
Are the leds on the case lighting up when you try to boot? Do you hear the fans spinning? Do you hear the hard drive spin up to speed?
Does the hard drive led flash for a while like it normally does when you are booting into Windows?
"My monitor goes to sleep when I turn the computer on .I have replaced the graphic card and also tried it with another computer but the problem still persist."
Clear as mud.
What did you try with the other computer? The replacement video card, the original video card, or the monitor?
Onboard video is a video adapter but IT IS NOT A CARD. Were you using a card in a slot originally, or onboard video?
When you have a video problem with a computer, usually there's nothing wrong with the graphics adapter, unless it has been damaged by something dumb you did, or the fan on the video card is no longer spinning, if that applies, and the video chipset has burnt out.
If you tried the graphics card that was in it with another working computer, and it didn't work, assuming you hooked it up properly (some recent cards must have an extra power connector from the power supply plugged into a small socket on the video card), the graphics card is probably fried.
However, if the other computer is recent and has onboard video, and did not have a video card in a slot before, some recent mboards DO NOT disable the onboard video when you install a card in aslot, and by default a video card in aslot produces no video! You have to go into the bios when the monitor is connected to the onboard video port and change the Primary video or Intialaize video first or similar setting to PCI-E or similar, Save bios settings, then the video card in a slot should produce video.
If the monitor does not work when it's connected to a working computer, the monitor is defective, and probably not your computer video.
If it's an LCD monitor, the backlight is probably failing, or alot less likely, the voltage inverter that supplies it with high voltage is defective.
For some info about replacing a voltage inverter or backlight, and what some typical symptoms are, see response 5 in this:
Most people just get another monitor.
CRT (tube) , Plasma, and the recently introduced expensive LCD LED monitor's displays last a lot longer than LCD monitors.