can’t login 404 not found

Actually, just upgraded to 2.71 but here is my issue.

I was in the General Settings section and decided it would be nice if I made the header clickable back to my main site. So . . . I changed the WordPress address (URL) from http://mydomain.com/blog to simply http://mydomain.com

Now I can’t login to http://mydomain.com/blog/wp-admin. I get a 404 error

The requested URL /wp-login.php was not found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

When I go to my site via the control panel, all the files are there, and the blog shows up (minus the template).

How do I correct this?

I promise never to do this again.