What happened to Manny Ramirez's power at the plate?

Many reasons. He's much older than he was. When you get old you won't be as good at things either. He is no longer juicing. That's big. He plays at Dodger Stadium which is not as nice a hitter's park as Fenway. He has battled injuries. And finally, he is getting thrown pitches down in the zone. Manny is maybe the greatest high-ball hitter ever, and pitchers have caught on, and aren't throwing anywhere near the top of the zone.