Google released a new update today (although reportedly only affecting 1% of results) that penalizes sites with ads above the fold. In other, words, the less content you have above the fold, the more you will be penalized.
The point of the release is to make it easier for search users to sort through the muck of advertisements and the hassle of having to push through ads to get to any real content. Since the update took effect today, the penalties also took effect, so theoretically your results should be easier to sort through already.
If your site got hit, you’ll need to fix the amount of ads, and then wait for the Google crawlers to come around again for any substantial righting to take effect.