I have asus p5p800-mx mother board morboard

How do you know the motherboard is working fine?

Please connect your system to another monitor and check if its working fine or not.

Problems can be in Data cable, Monitor also.

Have you tested the monitor to see if it is working? Is the vga cable good? Are you getting the BIOS start-up display screens when computer is booting up or no display at all? It is odd that both the on board video and video card would be bad I would double check the cable connection and the monitor. If the cable is good and the monitor is working properly than the board might be bad. You should at least get the BIOS start-up screen display.