Lost my site!

My site has been up for a while and now I lost it.

1) my site is set up = root > blog > index.php

therefore to get to the blog = domain.com/blog

2) I went into admin > options > general and changed the WordPress Address (url) = domain.com and all of a sudden 404.php

3) I reset to original domain.com/blog but my dashboard, it has turned to just content.

4) I can see the entire contents of my site = domain.com/blog/index.php but it is devoid of any theme, just content.

What did I do and how can I fix this? Thanks!