I think the key to this issue could be the usage of the PostFX.
Rhia474 mentioned that the render turned black after the render is finished or after the canceling of the render.
This sounds like the PostFX is on and working, right?
This happened to me a few days ago and it was a negative value setting for exposure.
I also had some difficulties to get rid of the setting, it jumped back to the negative numbers after closing the PostFX dialog.
To fix this I had to set a positive number and use one or both save options in the dialog.
Hope this helps.