This issue will occur if the Mailboxes and Folders menu has been re-sized and made so narrow that it is no longer visible on the side.
To amend this you will want to hover over the edge where the Mailboxes and Folder compartment would normally show and once you have the re-size cursor appear you can then drag this out to make the menu wider and visible again.
If you are still having issues with this, please raise a new online case with the support team anmd an engineer can talk you through the steps.