+1 on the same thing happened to a few sites we work on. We have now made all the changes we can to on-page and technical and will let that stew for a few weeks before we consider anything else. Some traffic has returned but there is a long way to go.
In the mean time spending non-ROI money on PPC. Ugh
Would sure be nice if the webmaster tool recommendations would keep sync with Algorithmic changes so you could properly react to all the silly black and white creatures.