Site rankings dropped from ~15 to 500+ but Google says we were not penalized

[theLotter]

2 months ago my site was ranking about 15 for my main KWs in the UK (my main market).  On July 28 we dropped suddenly to ~500 or not in the top 500 at all in the UK for the 3 main keywords.  However, these same terms are still ranking in other markets and we are doing okay in the UK for other terms targeted to the same pages.

I cleaned up my links/content and sent a reconsideration request to Google.

I then looked closer at my links and realized Google may be counting my affiliate links as if they were mine, so thought maybe that was the problem. I sent Google another reconsideration request and they wrote back the letter pasted below

Any ideas about what happened or how I can get my rankings back?  This definitely doesn't seem like it was just a simple algorithm change.  There were no major changes done to our site, and we are still ranking in other markets.

Dear site owner or webmaster  
We received a request from a site owner to reconsider your site for compliance with Google's Webmaster Guidelines. 
We reviewed your site and found no manual actions by the webspam team that might affect your site's ranking in Google. There's no need to file a reconsideration request for your site, because any ranking issues you may be experiencing are not related to a manual action taken by the webspam team. 
Of course, there may be other issues with your site that affect your site's ranking. Google's computers determine the order of our search results using a series of formulas known as algorithms. We make hundreds of changes to our search algorithms each year, and we employ more than 200 different signals when ranking pages. As our algorithms change and as the web (including your site) changes, some fluctuation in ranking can happen as we make updates to present the best results to our users. 
If you've experienced a change in ranking which you suspect may be more than a simple algorithm change, there are other things you may want to investigate as possible causes, such as a major change to your site's content, content management system, or server architecture. For example, a site may not rank well if your server stops serving pages to Googlebot, or if you've changed the URLs for a large portion of your site's pages. This article has a list of other potential reasons your site may not be doing well in search. 
If you're still unable to resolve your issue, please see our Webmaster Help Forum for support. 
Sincerely, 
Google Search Quality Team

[Tintanus]

Hi Sara, I am sorry for your loss. It happened to my site as well but it is finally going up again. You can take a look at this thread:

http://www.seomoz.org/q/amazing-decrease-of-visits-in-a-good-content-site

I hope it might help you. By the way, I finally noticed that the problem was in the alt attributes.

[theLotter]

Thanks for the thread, it was definitely an interesting read.  However, missing basic and/or more complex issues on the site doesn't seem like it would explain a 400+ geographically specific drop in ranking for specific keywords...

[Tintanus]

Yes you are right. I just wanted to help by making you see that your problem has in fact happened to many people. Mine is a bit diferent. To be honest, I still don't know why it happened and Google also told me it was not a penalization... Good Luck.