If a page has been built in Oxygen, you will see the option to “Edit in Oxygen” and the option to “Use default editor”:

If someone has clicked to use the default editor (which removes the Oxygen design) and you need to recover your Oxygen design, then this is possible by clicking “Edit in Oxygen” on the top bar:

Once the builder loads, you can restore the page to your Oxygen design by clicking the three dot menu in the top right, selecting History and switching to the Revisions tab:

Click a Revision to preview it in the builder. Once you’re happy that you have restored the correct version of the page, click Save.