g1smd - 7:45 am on Jul 8, 2011 (gmt 0)
If it isn't back in ~2 weeks, start looking for problems on the site. I've just seen a homepage drop to ~55 in a [site:www.example.com] listing for a week or two, and then recover. There were major changes going on in the site architecture while that was happening. It recovered several weeks ago, and is back at position 1.