I've seen some sites I follow drop to the second page. If your site historically sits at the top of the serps then if one were to judge by past experience, it's very likely to come back within weeks or even a month or two.
I'll agree with MB. This is somewhat common behavior with Google. Especially if there are major shifts in the index count. If you see pages drop, pay close attention to the number of pages being returned for that query. Usually it is quite a bit higher at the point your pages are moving. I call it Google Juggling.