Can’t access WP Admin section anymore!!!

Hi,

I’ve got my sister’s site in my hosting account so the real address for it would be http://www.mysite.com/sisters_site. She bought a domain and I simply had it point this url. Well, I went into her admin section to change the WordPress URL and blog address from http://www.mysite.com/sisters_site to her domain name, even though she doesn’t have a hosting account with files that point to that domain. Stupid, I know(I wasn’t thinking). Now I can’t access her admin section anymore and it’s screwed up her site(Stylings are gone and posts aren’t showing). Can anyone tell me what would be the best solution from here? Any help would be greatly appreciated.