I have been using a external monitor to extend my laptop display. However, just yesterday, my external monitor went black once windows starts. Im am using HDMI cable to extend the displays. My laptop screen works fine.
I have updated my graphic drivers and changed the HDMI cable but the issue still exists.
The monitor works fine when I connect it to my macbook.
Did you check that you didn't hit the key on the keyboard to switch displays? Different laptops have different ones, but it will often have two display icons on it.
Also, did you try connecting a different external display? While it might not be that monitor, it is possible that the computer itself no longer will allow connecting an external display. If the GPU has problems. Try connecting it to a different monitor, TV, etc., and see if they will work or not.