Yeah I know these are Hitfilm Forums, but I don't have a Blender account.
I rendered a scene with lights in it and models, and I saw it during rendering, but when I watched the rendered video, It was completely pitch black!
@georgefilmltd From your saying 'when I watched the rendered video,' am I correct in assuming you rendered straight to one of the video export files? I have always had a problematic experience with those and I just automatically render out of Blender in the PNG sequence format as a default although, there are times when an EXR is handy especially when render flames/light blooms etc on a transparent bkg for comping later in Hitfilm. It kind of sounds like Blender wasn't set to render the proper layer though, but without seeing the interface I can't be certain at all.
For video output check your Output settings:
Check Compositing Settings: Use Nodes
Based on your statement "and I saw it during rendering, but when I watched the rendered video, It was completely pitch black!" -- sounds like the Render Layers are not connected to a Composite node but I don't render videos from Blender rather I do like @tddavis does and render out png/exr. sequences. I don't render out of Eevee so this is my best guess.
@GrayMotion @tddavis Thanks! I'll try these out!