For testing I have several Gmail accounts. Recently, I noticed that one account has a nice sidebar:
But the other account has the mailboxes/labels collapsed and I couldn't see anything:
When googling the issue I immediately found some answer to click on the "icon on the upper left". Of course, there was no screenshot which is always annoying. It took me some hunting to find the correct icon. The writer meant the hamburger icon next to the Gmail icon above the mailboxes with the help text "main menu":
We are used to new style user interfaces where "main menu" doesn't need to have a menu, don't we? But I now have some mailboxes again in the second account.