You can’t rest on your laurels with SEO. You have to keep working on it, or the competition will bury you. 
EDIT: Hm, original post is gone!
Had you given your website, it would have been easier to tell you the solution.
I think you have been penalised.
-Duplicate / stale content (update with fresh content)
-Backlinks from junk / low quality websites (check for quality of backlinks)
-Involved in black hat SEO (reciprocal linking , hidden text etc…)
-Fluctuations in SERPs (happens sometimes but returns to normal)
-Check your Google Analytics account and pin point those keywords and pages for which traffic has dropped.
I would suggest you to track your keywords using Google analytics since it will help you give a complete report of keywords due to which your traffic dropped.
Spy on your competitors for their optimization strategies.
Either you are not working consistently or your competitors are working really hard.