Most notebook/laptop systems have a port to connect an external monitor so you need a way to control which is used to display the Windows desktop. For example on mine, if I connect an external monitor I can hit Fn-F7 to change from using the internal display to the external display. If I hit Fn-F7 a second time it displays Windows on BOTH displays. If I hit Fn-F7 a third time, it changes back to the internal monitor.
If I hit Fn-F7 with no external monitor attached, it sends the signal to the external video port, but since there's no monitor, all I see is a black screen. If I hit Fn-F7 a couple more times, I get Windows back on my internal monitor.
If you have such a "hot-key", you might try using it to see if your Windows desktop will cycle back around to the internal display.
If this doesn't work, the internal display or display adapter may have failed and you'll need to have it serviced.
In the meantime, depending upon the type of failure, you might be able to use an external monitor, if you have access to one, until you can have it looked at by a technician.