A few days back there was a Google algorithm update.Which is why all of us have seen these drastic changes in search results.
My suggestion is if you want to get your raking back, Build pr3+ backlinks with your target keywords as anchor text. Even i lost my rankings tooo.
Frankly speaking i was getting decent traffic for 25 keywords on my site, now only 3 of them work, Thanks to yahoo, I got a hike in ranking out there, & guess what my adsense revenue has doubled since i have started getting some good traffic from yahoo.