Previously imported photos are no longer visible in Lightroom Classic

A likely possibility is that when you imported the photos into LrC, you accidentally imported the photos into a different folder than you expected them to go to. This has happened to many other people. Your symptoms that Synchronize folder fails to find the photos indicate they are already in the catalog. So, you need to find these photos in Lightroom Classic. Please follow all four of these steps in the Lightroom Classic Library Module, in order. Let us know if this finds the photos.

1. Click on All Photographs (its on the left under Catalog)
2. Turn off all filters (Ctrl-L once or twice)
3. Expand All Stacks (Photo->Stacking->Expand All Stacks)
4. In the Filter Bar, search for one of these photos by File name

These photos may be stacked, with another photo on top so you don’t see them and they are not counted.

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When I go to Synchronize folder, it tells me 16 are missing and when I hit import in the Sync dialog box, it says no new photos are there.

Synchronize Folder has been buggy for a long time, incorrectly showing missing photos:

The only reliable way to see missing photos is to do the menu command Library > Find All Missing Photos.