Hi accounting30,
Thanks for joining us in community! There's been an update to QuickBooks Online regarding the options for the left menu. Don't worry, I'll explain how to get those three "sandwich" lines back and why they're missing.
Previously, the three lines used to appear at all times, giving you the option to hide or show the left menu without changing anything. With the update, the sandwich lines won't appear unless your browser or app window is small enough that the program thinks you might like more room on your page.
To test this, click the Restore Down icon on your browser or app if the window is maximized. Next, hover over the side edge of the window until you see the horizontal arrows. When you see them, click and drag the window to make it narrower until you see the sandwich lines. Here are a couple of screenshots so you can see what I mean.
Wide Window
Narrow Window
Notice how you see less information on the narrow window? That's an instance where you might want to hide the menu in order to see more details on the screen, so QuickBooks gives you the option here.
Give it a shot and see if you're able to make your sandwich lines reappear. Let me know if you have more questions. :)