When did they disappear? If it was about a month ago, all the servers got rolled back. However, some people mentioned that their thralls disappeared again after the rollback. You can check out a few reports of that happening on the forum here.

As for the thralls that fall through the ground when you knock them out, that’s a long standing bug. Just leave the area or log out and log back in and they’ll be there. They just need to rerender.