aakk9999 - 12:57 am on Feb 25, 2013 (gmt 0)
How big is the site and how long ago have you made URL changes?
It normally takes some time for Google to recrawl the OLD pages to see redirects and until this happens, a drop is something to be expected more often than not. After all old pages have been recrawled, then the site may recover or even result in better ranking, depending what the new URL structure fixed.
It would however be better if you did changes in two steps: first change URLs and then when rankings settle, then change page titles (and improve page content at the same time)