Hello, EtherealAccounting. I experienced something similar recently and tried to get some help from Copilot. The LLM was not helpful, but working the problem led me to notice that a recent Office update had change the Navigation pane default to collapsed. If you look in the upper right corner of your navigation pane and you see a horizontal push-pin icon, you can click that to make the pane remain open.
My experience was just another in a long, sprawling chain of UI changes that inhibit my productivity. I think that the root cause of all of them is a very poor decision at Microsoft (and many other software producers) that we need to have 'a seamless experience across devices.' This silly notion could only come from people who only work with a few files/messages on a given day and don't do much more than look at them.