New theme changed ranking? Help

I don’t know if this is possible, but I think my new theme ruined my Google ranking. I have worked hard to get a regular readership, and for many relevant searches, my sight was coming up on the first page of Google. I changed my theme, and now I am no longer on the first page for the exact same posts that were on the first page before I changed themes. In fact, I have searched for my posts, and they are now so far back in the Google listings that I gave up searching before I even found them.

How did this happen, and what do i need to do to get my Google ranking back?

I am using All-In-One SEO Pack. I was using it on the old theme, and I am still using it on the new theme. Could the new theme have disconnected All-In-One SEO Pack? Please help.