It means that source is located outside of Preview area or source hidden (source may be hidden by the filter that no longer works properly or via the "eye" icon in Sources list of OBS).
Right-click menu over the selected source will give you few options to change
Transform (position, size and cropping) of the source. Reset the transformations to default and source will become visible.
Make sure the Preview is not zoomed and panned out of your view. Right-click menu over the Preview, check
Preview Scaling > Scale to Window
Remove not working filters from the source (right-click menu
Filters) and source will become visible.
Similar threads over the time:
Hey, ' This is NOT the same black screen issue many have been dealing with, so please dont just refer me to the fixes for that. I have a black preview screen when attempting to stream Rainbow Six Siege in fullscreen, but when i right click game capture, it shows that the game is in fact being...
obsproject.com
its not just that my screen is black, I was moving around the screen, y'know experimenting, trying to understand the program, when I dragged the screen to the left, out of view and out of reach. I thought I might have just left it over under 'sources' so I could grab it again. But I tried...
obsproject.com
I realise this is a tediously common question, but I've no where left to turn. I installed OBS on the recomendation of a friend who said it was good for game recording, but I can't get a recorded image no matter what I do and I just get a black screen. We both use Windows 7 (64Bit) and the...
obsproject.com
and many more.