If the computer is going through POST checks okay without this card installed, but fails POST checks with the card installed, my first inclination is to suspect that the power supply is lacking sufficient oomph to properly power the graphics card.
Second, check that the graphics card is properly installed, aligned and seated in its' socket.
Third, check you have the correct and most recent drivers installed and BIOS settings are correct for the card. You may have to use a VGA style monitor to download and install the drivers before installing the new graphics card, or switch graphic cards from the BIOS from motherboard to external card. The fact that you refer to VGA on the computer leads me to suspect it is an older model.
If the card is not brand new, it may have a fault. If it is brand new, the power demands may be more than an older computer can supply. It could be faulty too, but fairly unusual in modern pieces of kit.
If the monitor has various user adjustable settings with an on screen display, check the correct input and display settings are set up. The monitor itself could be sending a signal back to the computer indicating that it is not prepared to play ball.