This only happens to me sometimes, not always, after I delete lights and put new ones in. I was really disconcerted the first time it happened. I went ahead and did a 1/4 size render (which showed up) and was pleasantly surprised when all my stuff appeared where it was supposed to after that. I can't figure out why it happens sometimes, and sometimes not, but most of the time when it does happen it's because I've been deleting lights and clicking tabs really fast. It might not be that for sure, but that's been my observation.