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?! :(
It appears that you have hit a review ban - too many bad reviews (i.e. - rejected reviews).
This will expire in about an hour or so - be sure to review more carefully ;)
You had too many bad / rejected reviews. This oftentimes is the result of failing the one's Stackoverflow places in the review queues to test your attention.
To see when your ban will be lifted visit the Review Overview page.
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.