While I was working on my scene I, decided to give it a render, and it was all going really well but just when it was about to finish Rendering, whole the render Image turned black.
Just like that: !
the compositor is not getting any input, I think. you can connect the "image" output of the rgb node ALSO to the compositor, which builds the output. Just drag it.
you can leave the viewer connected, to see the result of that (or others) node output, too, like the backdrop, if you use it.