|First of all, I don't think you can mistakenly disable the onboard graphics. It is the default video. |
You can reduce the amount of RAM dedicated to graphics but if no add in card is installed the onboard whould automatically work.
Conversely, if you install most types of add in cards it will automatically disable the onboard graphics.
As jam stated above, you may not have the card seated correctly. If you installed a full height card instead of a low profile card, if required, you may have the card half pulled out of the slot. That could possibly cause permanent damage to the motherboard.
I don't know if that is what you have done or not.
Unplug the computer and remove the add in card. Connect the monitor to the onboard and reboot to see what happens.