Site penalized for a single keyword?

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My site used to rank for 'hcg diet' 1 year ago. Aft
er the Google updates it nosedived and completely vanished from google inde
x. The site is ranking for other keywords fine but seems to be penalized fo
r only this keyword. I have not done any link building at all for this site
 - if you look at the site you will find hundreds of articles - all origina
l written by a staff writer, and the site ranked on the strength of its goo
d content only. No manual link building was done in last 1 and 1/2 years.

I also filed a review request and the Google team checked my site and maile
d me that all looks fine. Still no ranking for that keyword, while useless  
1 page websites and illegal drop seller sites are ranking for that word. Wh
at can i do to get back rankings considering i do not have access to their  
management to request a review again?

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