You can press "Z" on your keyboard to toggle open the sidebar (or "X" to toggle the Plan Day sidebar). You can also hover with your mouse in the middle right of your screen and click the sidebar icon to open it.
Finally, if you have "Show Marvin logo (click to collapse)" setting on in the sidebar strategy settings, you can click the small M at the top-left of your screen to expand it.
Why was my sidebar closed?
The sidebar stays collapsed when you collapse it using any of the above methods, even if you reload the page. Additionally, the sidebar will be initially closed if you open Marvin on a device with a narrow screen, such as a smartphone or a tablet in portrait mode.