Changed Theme – Missing Posts

I set up my site today, added 49 posts and a link, decided to change the theme. Now I can’t find any of my info. It says the posts are there, but when I view I get 2 posts, no categories and no second column. What did I do wrong and how do I fix it? Do I have to delete and start over and post anything or is there a way to recover. I’m new to this, so don’t get too technical. Thanks