Okay - not all computers have a graphics card that's removable. Some just have a graphics adapter permanently attached to the motherboard. In both cases, it's where you plug the monitor in to the PC case at the back. You'll know which yours has as soon as you open the system case.
If yours is permanently attached and it has failed, you'll have to install a graphics card in to one of the PCI slots, then plug your monitor in to that.
It sounds to me like that may be out of your depth so get someone to check it out for you and repair as necessary.