Happens many times througout the year. Not much you can do about it. Generally sites come back. LOL, evertime I seem to lose rank in Google, I get a ton of new Yahoo keyword hits. Sadly, the Yahoo traffic is not comparable to google traffic.
my site on google.de is back on #1 again, where it deserves. another site (content rich and quite new) has also gained better rankings. seems to be that sites with content will be more preferred than before or an overall mayor update, which was probably done like that before, has taken place.