Computer booting but no display

When I turn on the power of the computer, it boots up but nothing is displayed on the screen. The graphics card is replaced but still no display is found on the screen.
I also try to reset the BIOS by removing the battery in CMOS. I try to remove the Random-access memory i.e. RAM and reinstall it and get the beeps. But there is still no display.
Any one could help to figure out what is going on? Many thanks.
