I had this same problem for several hours today for all my books. I checked because my sales were way below their normal range. I wrote to Amazon but haven't heard anything back yet. The problem, though, seems to have resolved itself. But I took a bath today with far lower sales than normal matched against a normal-level ad spend that sent clicks to pages with NO buy box. Maddening 
Actually I think something like this happens to every single page on a regular basis. And has been for years.
There are always unexplainable days when your sales and reads drop like a switch was turned off, and when you look, it's like most of your books were not actually there for most of the day.
It happens regular as clockwork (but the clock is broken), and never fails to knock your momentum off it's stride when you get some.
I swear it's a deliberate thing Amazon does in order to stop people getting too many up days in a row.
That it might be a bug which mutilates the product pages for an author for a few hours a day, every week or so, has occurred to me. If so, it's something that's been around for donkeys years and they've never fixed.
