your site may drop in ranking for several reason. lack of attention, too much competition, backlinks disappearing, and many other reasons. i usually do mixed link building strategy and dont stick with one single link building method. its just more natural to use several link building methods. i do dir. submission, social bookmarking, blog commenting, sometimes article submission, buy links and blog posts/reviews, used to do press release but havn't done that in a while, forum sig link - thats all i can think of now
I guess be patient and wait and if there was something you were doing before and stopped maybe you need to get back at it again. Like Link building for example.
Most of website are dropping right now as Google just made a big update of algorithms and something called "Flash Indexing" so always need to build fresh links and add new content, but be careful where you add content as replacing old content always hide some risk.