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.