First thing is to have a long hard look and see if there are any technical issues that might have caused the drop. Be sure your pages are all reachable by Google, etc. You want to rule that out before you look to the algorithm being the problem
Then have a read of this thread before making a lot of changes [webmasterworld.com...]
If the drop was on a single day, then you'd be looking at Panda as a possible culprit. Last update was July 24th, so check if that roughly coincides. Unfortunately just focussing on content / innovation, etc. doesn't mean they won't sting you.