Syncing is the wrong thing to do. You need to find out where they really are, and then 'relink' them: https://www.lightroomqueen.com/lightroom-photos-missing-fix/ One reason why images could become missing is if they are on an external drive, and that drive is either not connected or Windows assigned a different drive letter to it than it used when the images were inported.
In addition to what @JohanElzenga said, some operating systems sometimes allow (it's a bug) you to ADD photos from the camera card, and once the camera card is removed from the card reader or the cable from the camera to the computer is removed from the computer, you can get these same symptoms of photos missing that were just imported.
If you import from a card make sure to always use the Copy method. Copy should be highlighted at the top of the screen during Import.
It's important to understand the difference between Import Add and Import Copy. If you don't you will eventually lose track of your images. Many beginner Lightroom Classic users misunderstand how import works and it causes lots of problems for them down the road.