Everything seem to be working as usual except for the c:\ root folder. It has worked for many years. I don't know when it went away. All drives and folders show properly except for c: root. It shows no files or folders in file pane, but tree tree pane is fine. I have tried changing the folder setting in TOOLS > FOLDER OPTIONS. My various toolbar filter buttons still work as always in all folders and drives except C: root.
The C root folder shows up properly in Windows Explorer, but not in DO.
Adding the hidden item count to the status bar did reveal that everything was hidden. I had checked the folder option SHOW and HIDE tabs. I had thought the the applicability referred to the drive or folder, not which side it was shown on. However, after more trial and error C: worked on the left side, but not the right. I finally got it to work on both. I have not idea how it cont changed to begin with.
Folder formats/options are usually tied to the folder you're looking at (and you can see a list of those formats under Preferences / Folders / Folder Formats).
But they can also come from the layout, style or folder tabs you've loaded. (Including the Default Lister, which is really just a special layout.) Those things all have options to ignore their folder formats, which should usually be turned on to simplify things, except when you actually want them to override your usual formats (e.g. you might want a layout or style that always uses thumbnails mode).
If the format lock is on then it can get more confusing as well, since the lock makes format overrides stick even after changing folders.