I had a similar problem some years back when most of my Poser formatted items disappeared along with some of my Daz stuff. I didn't do anything, it just 'happened' one day. My graphics computer doesn't go online so I know it wasn't a virus or some strange web glitch. I re-imported all my content folders and got everything back, although to this day Studio can't find some textures for a few Poser items even though they're obviously where they're supposed to be.

My suggestion is to....darn...My screen capture tool wont grab the drop down box so I can't show you. In your content pallet go to the far upper right corner and bring down the drop down box with selections for 'View as list', view as tree', and so on. There's numerous tools in there for 'scanning known directories for files', 'migrating old content', and the 'content directory manager'. Try any and everything there, this worked (sorta) for me. From all the constant complaints I see in the Daz forums and the Discord servers, this is a known problem with Studio. Let us know how it goes.