mememax - 10:41 pm on Dec 9, 2011 (gmt 0)
Have you changed only your layout? The content is exactly the same? Code is the same (or better optimized)?
Consider that even if you've ranked great during the last 5 years, it doesn't mean you've done everything fine and maybe google is penalizing you for a mistake you did few weeks ago.
It's possible that Google crawled your page and found it was highly different from the one it has in its cache. This will launch a reranking process (it's my speculation). If this is so you should recover rankings in a week or so.