Opposite URL changes by client.

I am a designer, and this problem has restricted access to the admin, so I don’t know how to solve it.

The installation of wp was originally done in http://www.domainname.com/wp. The client has changed the URL of the blog (via the admin) to http://www.domainname.com/wp and the wordpress URL to http://www.domainname.com which is causing an internal server error.

Otherwise everything is intact. This relatively minor error has caused a major problem in that I can’t get to the admin anymore.

Suggestions?