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Give it 4 weeks. I can almost guarantee you will pop back into the results and in the exact same positions as you were before.
We believe it has something to do with the Google Datacenters. Maybe they switched the datacenter you pull results from and now you are pulling results from a datacenter that does not have you listed in the index.
We can't be sure because our client's experienced the same thing but their pages WERE indexed, just no longer ranking. 2-4 weeks later, they reappeared, in the same exact positions they were in before.