Images are some of the most important pieces of information on the web, but over the web’s 25-year history, they haven’t been very adaptable at all. Everything about them has been stubbornly fixed: their size, format and crop, all set in stone by a single src.

Did you do 301 redirection to each and every URL?

Did you downloaded all the URL’s by signing in to your FTP or Cpanel account?

Did you change any content, meta tags or onpage elements?

Login to your google analytics account and export All Pages under site content. Compare the pages and find out if the pages are same for before site migration and after site migration is completed.

How many 404 errors did you receive in your google webmaster accounts. 404 errors will be generated if user’s click the url’s from google search or links present any where on the internet. Try to create an alternative page or redirect that to some other similar page.

You are saying that your traffic dropped from 1000 to 150. Did you checked which keywords ranking dropped, remain constant or improved, what is the landing page and ranking. 150 traffic that you are receiving are for the same pages that used to rank before or are new pages.

Try to find all the landing pages that used to rank for your important keywords, map your keywords to that URL and also try to add informational keyword rich content on the landing page that used to rank before website migration this may help in improving your rankings.

For more detail information read this Web Site Migration Guide - Tips For SEOs