When you are working on your test blog, it's a very good idea to not have your real blog open in a window or on a tab. It's very easy to accidentally click the wrong window or tab and delete the wrong blog template.
Ask me how I know...
Also, always keep a back up of your blog for such times when mistakes take place. I am VERY glad I did. :o)
Now, go try some new things to do with your blog. Blogger Buster is one of my favorite places to go to for tutorials and tips.