I found a workaround that may work for you.
As others already reported, first go to https://new.remnote.com. In my case, it loaded most of the data but not the left sidebar. Try to create a new folder using the button in the bottom left. In my case it caused the other folders in the left sidebar to appear again.
As for the desktop app, try finding one of your top-level folders using the search box, click “…” in the top right, and “change document status” to “pinned”. In my case, it caused the “pinned” folders to be visible in sidebar again. This has to be repeated for all of the top-level folders.