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

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.

#1

You could disable Use Nodes. When this is disabled, anything in the compositing node tree's are ignored.

#2

Or you could add in a composite node, and attach it to the Render Layers node.

ShiftA output -> composite.

And then attaching the render layers node to it.

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

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, then continue.

Now there are two ways you could solve this.

#1

You could disable Use Nodes. When this is disabled, anything in the compositing node tree is ignored.

#2

Or you could add in a composite node, and attach it to the Render Layers node.

ShiftA output -> composite.

And then attaching the render layers node to it.

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

Select the composite node tree button (highlighted left). If you have a composite node attached to the render layer node, then this won't solve your issue.

Also check if Use Nodes is enabled. Disabling it should take care of this, but it's not as thorough, and if you come back to do compositing, you'll want to add in the composite node anyway.

I would recommend just adding in the composite node with ShiftA then going output, and composite.

And then attaching the render layers node to it.

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

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.

#1

You could disable Use Nodes. When this is disabled, anything in the compositing node tree's are ignored.

#2

Or you could add in a composite node, and attach it to the Render Layers node.

ShiftA output -> composite.

And then attaching the render layers node to it.