What's going on all for sudden drop of keywords

For the past 2 weeks my site work well for the keyword accounting and other accounting related terms. Then suddenly all gone from the top 50 search, I don't believe my site have been ban because the total index page keep increasing and I see yahoo bot still crawling my site.

The site have more than 25K back links in yahoo.... any one can tell me what's going on... is there a way to bring back to the top.

 

Is it in a competetive field ?

AFAIK, yahoo has a sanbox and that is causing the drop now a days.

 

Thanks.... maybe you're right it cause by their sandbox..

 

Yahoo and Google always seem iffy. Msn is seemingly the only constant directory in terms of rankings.