Cannot access my blog anymore

I screwed something up and now I cannot access my new blog anymore. Background (some of this may be important, some may not):
My site is hosted by Bluehost.
When I installed WordPress, I installed it in the primary directory (first mistake).
I wanted it in a subdirectory so I could work on it before it went public. I asked Bluehost how to move the blog into a subdirectory, and they told me to first update the URLs in the dashboard of wordpress.
I went into my dashboard and updated the URLs by adding “/blog” to both of them.
Now, when I try to access my blog, I get an internal error.

Obviously, I’ve made a mess of this. Any help would be greatly appreciated. Thanks. Peter