Matkat, an index page go missing seems like a perpetual problem with ATW. I have no good idea why that is happening in some cases, but reports have been made here frequently.
It does however not seem to be any penalty but rather a technical thing. From what I have seen so far the pages usually reappear in time. You might want to give it some time. Else I would contact ATW through their webmaster contact channels on their site, they have been known for excellent response.