No preview image showing up when I plug the Viewer node in. I read elsewhere that I needed to render the image first. That worked for a little while but now it doesn't work anymore.
For future readers (like myself), also make sure you have actually rendered your layer(s) first. The compositor does not render them for you.
You will know you have a rendered layer when you see the thumbnail of the “Render Layers” node, just as you see in jachym michal’s answer.