It's typical for this sort of thing to happen when you make a large change to a site, especially if you change everything about the site, especially the subject of the site.

I doubt you've been sandboxed, Google just decided that your site has changed so drastically that they have placed you into a "time out" of sorts so that their algorithm can review your site a bit more thoroughly and prove to itself that it is ranking your site for the "correct" terms. Do a quick search on Google for your domain name and if you appear than you have not been sandboxed. Normally within a few weeks to a month you'll see yourself start to move back upwards in the rankings, just keep moving forward with your current strategies.

This penalty or "time out" was implemented due to an old practice of people getting a site ranked #1 for something like puppies, and then suddenly changing the site to one giant viagra ad and I'm doubtful we'll ever see it go away. A bit of time and you'll be fine.