from your file manager (which isnt showing you any files)
view tab >
options button on the right side > opens new box Folder Options
view tab > restore default settings
select show hidden files
un-select show icons instead of thumbnails
apply.
Should your thumbnail cache be corrupt > follow this sites instructions
https://www.windowscentral.com/how-reset-thumbnail-cache-windows-10
I hope that solves your issue. (just check your active programs, maybe one of them is trying to force you into window for dummies mode - eg everything turned off kid friendly).
Adding folders to your library should be done from the library box >
right click library folder >
select restore default libraries
if the libraries you want are not included in the restore, you must add them by>
open the library name and select add location >
navigate to location of library folders you wish to add to master library.
eg pictures libaray is restored, but no pictures = open pictures library > add location > navigate through your HDD to where your pictures are located.