Do you mean when you try installing the drivers, it says "graphics card not detected"?
Because drivers don't get "detected", they get "installed". You don't detect drivers, you install them. After installing them, the system knows how to run the installed hardware BUT for practically any modern graphics card Windows should already be able to supply a generic universal drive for it, which means it has already detected the hardware if you have a display.
I'm willing to bet you did not move the display cable from the motherboard video output to the graphics card video output. Or, you did not connect the required PCI/PEG supplemental power connector from the PSU to the graphics card.
What is the EXACT model of the graphics card, motherboard, CPU and power supply?