Check out your backlinks whether they are still there or got removed/expired. Secondly do a competitor analysis - on page and off page both. Note down the changes they have made in their website and check whether they have received back-link from some new sources or so.
I tried answering your query but thought Google’s Matt Cutts can explain it better. Here’s the man himself talking about process that you can take to diagnose drop in your rankings: http://www.youtube.com/watch?v=gx04qNrCbnk&feature=player_embedded
check your links and then look if any sites are banned from google - if you are on a site that is banned - remove your link - you can also sign up webmaster tools and then ask for reinclusion...