A couple of Samsung owners have reported this over on Twitter and one or two people here on the Community Forum.
I don't own a Samsung Smart TV with the native App, but i would try unplugging the power to both your TV and Router for a minute and restart your Router first (give it say five minutes for it to settle down and then plug the power back into your TV.
If that doesn't work, maybe try a sign out and sign back in on the App or a fresh App download from your TV Apps Store.
Going back a few years ago i temporary lost my Watchlist for a day or so, but it reappeared without my input.
As a worst case scenario you could set your TV back to factory settings.