Thanks for reaching out with your question here in the Community. I'll help explain what happened to the 3 bars for collapsing the left menu, and how to bring them back.
We've received similar questions recently regarding this issue. QuickBooks Online has been updated, causing some changes to be made to the interface. Take a look at this article for more information about why the collapsing menu option isn't showing up, and how to bring it back: 3 lines no longer showing up.
This article has all the information you'll need, as well as helpful screenshots to get you back on track.
If you have any other questions, please let me know.
I'm glad you were able to resolve the issue on your end. I appreciate you sharing your thoughts about the new update, and I'll be sure to share your feedback with my team.