render output is just completely black

I have a scene where it is well lit and rendered just fine until yesterday.
I must have accidentally messed up a setting at some point because today, it render completely black.

Once I hit F12, it renders for a mere blink of an eye and the outputs a completely black image. It doesn’t even begin to count the number of vertices, faces, layers or memory as it customarily does at the beginning of a render (as displayed in the upper left hand corner of the screen).

I’m sure it’s something completely stupid I’ve done.

Has anyone run into this before and/or have an idea as to what might cause this?

There are many possible causes for this. Best to post the blend file.