You've got me here to help you retrieve the missing left navigation bars on your homepage. Pressing the Hamburger button can hide the bars at the left of your screen. To help verify if this button isn't accidentally clicked, I suggest ticking it to bring back the navigation bars.
Please see the screenshot attached for reference:
Once done and the problem continues, it could be that the stored data on your browser be the reason behind the missing bars in QBO. To fix this, I recommend accessing your account using a private window. This prevents the browser from collecting cache files (cookies) and allows you to utilize QuickBooks with a fresh slate.
To do that, just press on the following keyboard shortcuts below:
Internet Explorer: Ctrl+Shift+P
If it works fine, go back to your regular browser and clear its cache. Web-based platforms like QBO gather cache to save you time when loading repetitive data, but too much can create odd behavior and simply needs to be cleared.