The issue is most likely that you removed the composite node accidentally in the composite node tree.

[![enter image description here][1]][1]

Select the composite node tree button (highlighted left).

At this point, we should take a step back, and make sure that what I think is happening, is happening.

There are two signs that I'm wrong:

 1. Use Nodes isn't enabled
 2. There is a composite node

If both of those aren't true, than continue.

Now there are two ways you could solve this. 

<h3>#1</h3>
You could disable Use Nodes. When this is disabled, anything in the compositing node tree's are ignored.



<h3>#2</h3>
Or you could add in a composite node, and attach it to the Render Layers node.

<kbd>Shift</kbd><kbd>A</kbd> output -> composite.

[![enter image description here][2]][2]

And then attaching the render layers node to it.

[![enter image description here][3]][3]


  [1]: https://i.sstatic.net/Ic8rx.png
  [2]: https://i.sstatic.net/iikSu.png
  [3]: https://i.sstatic.net/I6n9k.png