The first thing to check is if your Display Adapter drivers have suddenly stopped working or if you are using a NVIDIA Gforce graphics driver. I've had nothing but problems with NVIDIA's drivers. So I disable it always but sometimes due to windows updates it gets enabled again. This was my problem after windows did an automatic update today. All live streams and playback of my cameras displayed a black screen, yet there was sound and the cameras were connected and still functioning, I could even access all their settings and make changes just not able to view the stream. I'm using windows 10. Right click on bottom left corner start and choose Device Manager. Look under Display adapters and see if any of the drivers listed has a small error icon. I disabled the NVIDIA Gforce and my other adapter which is Intel HD did have an error. Simply disable all adapters, then reenable the standard Intel. Fixed my problem Immediately.