Sorry to hear you're having trouble after your migration, it's all too common to see this happen when sites are migrated or new sites go live.
The most important thing is to make sure all old pages have been 301 redirected to the relevant new page / if no relevant page to the homepage.
The drop in rankings could be due to backlinks pointing at pages which now 404 - if you haven't already use a backlink explorer such as Open Site or Majestic SEO and make sure none of your incoming backlinks are 404's, if they are get them redirected asap.
Has the site moved to a new domain or is it just a relaunch?
Also have a look at this article on site migration - http://moz.com/blog/web-site-migration-guide-tips-for-seos
Danny
