Green_Grass - 2:06 pm on Jul 12, 2010 (gmt 0)

1 = death

Normally a sudden drop is bad news. Hope yours recover. Little you can do..

1 = normally a human review by the LPQ team deciding they don't like your landing pages.

You need to wait a bit and check what happens to the QS scores. The problem is that Google seems to subjectively, retrospectively apply some new guideline to an old account and the poor advertiser has no clue.