This has appeared as the dropdown on my review queue button today, and I can't find out anything about it.
Anyone know what it is? What it means? How long it's going to stay there?! :(
I discovered this week that there is one more situation where this happens: if you exhaust your daily vote limit. As soon as you do this, the review queues close, at least the basic ones that are available to lower-reputation users. I suspect this is the case because one of the preferred tasks you can perform in most review queues is to upvote or downvote posts.