Can't access the dashboard/back end of site

I’ve just logged into the back end of my wordpress site, and was having trouble getting my ‘Home’ page to show up instead of a filler “website under construction” type page a friend had put up for me. I messed around with url settings in the back end (not permalinks but something to do with two web addresses: http://www.[mywebsitename].com vs http://www.[mywebsitename].com/wp).

Now I can’t get into the back end at all, and when I try to sign in or navigate anything I’m getting a Not Found 404 message. This is what I get…

Not Found
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.