Changed domain- now locked out of my site!

In General- Settings, I changed the name of my site, and the domain name stupidly. I now cannot get in to my back office to fix it, and if I look up my website it has lost the theme, is just all white.

Please can someone help me. When I go into the log in page and log in, with Google chrome is just takes me to a blank page, with internet explorer it says ‘this content cannot be displayed in a frame’, if I try to open it in a new window it just goes back to the log in page. If I try to open it in Firefox, again it is blank.

What do I do???