When a graphics card is installed, it will be the primary video output (not the motherboard).
Does the tower itself appear to power up? Can you hear fans or drives spinning and/or do lights come on?
Make sure that the monitor is set to the correct input.
If you have another VGA cable, try using it. This will tell us if the problem is the VGA cable you're using. Don't try to make VGA cables bend sharply as the wires inside may break.
Please make sure that the graphics card is seated correctly (take it out and put it back in).
Also, if it has 6-pin or 8-pin (sometimes called 6+2 pin) connectors on it, make sure they are connected to the power supply. If a graphics card has these connectors but they are not hooked up then it will not function correctly.