How would i go back to my video card that came with my computer?

i have an emachines t5082 desktop. i have a nvidia geforce 8400 GS video card and lately it has been failing. i want to go back to the video card that originally came with the computer. i have the drivers installed and the port to plug the monitor is free to use. do i need to take out the current video card. i tried to plug the monitor up to the original port before i got my video card but the display didn't work. how can i do this?|||If you have a video card slotted into your motherboard the motherboard's onboard video card will not process any of the data. In order to go back to the onboard video card you need to remove the 8400GS currently in your computer.