There are a few possible reasons for this to happen. First, make sure your monitor is connected to the new video card. Turn on your PC and keep an eye open on the monitor LEDs, when powered and connected properly they should turn green of just be green colored (depending on the monitor model). If green, it means the connection is good. From here you should see the bios screen appear and do its normal system load and checks, then boot into the OS. Once in the OS, go to your device manager and disable the onboard graphic card.