First thing to do is check in the BIOS at boot. Some older MBs require you to manually disable the on-board video and enable the PCIe video. Modern boards do that automatically.
1) What is the make/model of the card?
2) Are you connected to the monitor the same way you were with the on-board?
3) you didn't forget to move the video cable from the on-board to the new card, did you?
I actually don't know where to find the video cable, to exchange it. I know about how to run pc's but not how to fix or upgrade. could you help me??