title: Table of Contents disappeared in Jupyter Notebook
body: <p>Currently, a jupyter notebook does not show the lateral table of contents. All the other notebooks do.
  In fact, for that notebook under <strong>menu->edit</strong> the <strong>nbextensions config</strong> disappeared, so, it looks like that notebook does not load the extension.</p>
  
  <p>My jupyter version is 4.4.0.</p>
  
  <p>How to fix that notebook?</p>