Lost my website traffic from search engine – Changed my website WordPress theme

There is no need to install analytic – yuck…
Better go with some seo toolbar, or there are also many on page link analyzers.
A agree that it could be the backlink problem, as mentioned.
But, there is another factor that is measured by search engines, ‘stability?’ – you call it… In fact, the less your page changes, the better authority you get in the eyes of google.
So, if your site suddenly changes, there might be a hit, but as long as you have the same link structure, it should recover itself with time.
If your structure changes, you would need some more time to regain authority…
Cheers.

might be some google update hit your site badly

While changing theme we have to be very careful with site structure and heading tags compatibility with the old theme, because it affects seo. You have to wait for google re-index your site.
And check for 404 error pages in google webmaster fix it.

I don’t think a Google update had hit your blog. The theme you’re using now may have bad coding or disturbing your SEO plugin. In my blog when I changed theme I got increase in traffic, why? because it’s seo optimized. But your new theme may have problem. Try reverting back. That may work.