Non-interactive notebooks runs inside a container on a server and uses server resources. To maximize the resources' usage, we ensure that containers which are actually being used are the only ones kept alive. This means that while you're working on your notebook, the container is kept alive as long as you are working on it. If for any reason you stopped working on your notebook, we will open a 5 minutes window after which the container will be stopped.
At this point you will get an inactivity screen in your Preview Pane that looks like the one below. Also, the controls will be hidden while the inactivity screen is still showing.
You will need to reload the page you're on to restart the container. You can do that by clicking on the Restart button that appears in the inactivity screen, or you can reload the page from your web browser control. Once that is done, the notebook will restart and the preview will show again.
Premium notebooks have a longer timeout window of up to one hour.