There are several reasons that your PC isn't detecting your graphics card, and therefore, the screen does not work. First, if you're using a desktop computer, open the case and ensure that all connections are correctly fitted. A loose contact point can cause hardware failure. Second, ensure that you have the proper graphics settings. You may need to disable the onboard GPU and reset it. Third, you could have a power supply issue, so check that your computer pulls enough power to run the graphics card. If you've recently installed or upgraded your graphics card, you'll need to install drivers for it. Aside from these options, you could have another connection issue, maybe Windows needs a software update, or you have a faulty graphics card or motherboard.