Nobody here can tell you why the albums are empty, I'm afraid that's something only you will be able to establish. However, there are various ways in which that situation can occur:
1. The user removes all photos from an existing album, either all in one go or more gradually over time. The fact that the album still exists with a zero count means the album itself wasn't deleted, just the photos removed from it.
2. The user deletes photos from the cloud using one of the Lightroom apps. Obviously, it an image is deleted from the cloud it will also be removed from any albums that it was in.
3. If the user syncs the cloud with a Lightroom Classic catalog, then either removing a synced image from the Classic catalog, or more simply removing a synced image from Classic's All Synced Photographs collection, will cause the image to de deleted from the cloud and thereby removed from any synced album or collection that it was in.
4. It's also possible to remove images in Classic from a synced collection (same effect as removing from an album in Lightroom), i.e. image is removed from both collection and equivalent album, but remains in the cloud.
Given the above, you need to figure out if the images that have been removed from the albums are still in the cloud. If they are, you can re-add them back to the albums, but there's no automated way of doing that. If they are not still in the cloud, then they have been deleted and (if that deletion was less than 60 days ago) you should be able to find them in the Deleted section. They can be restored from there, and if they were still in the albums at the time of deletion, restoring them should also restore them to the respective albums.