Admin Login Does Not Work!!—-HELP

When I go to login as site admin, it displays “Page Not Found”. The only way to log in is to sign my name/password, direct to the “Page Not Found”, and then go back to the homepage and click on “Edit” to get into the admin panel. Can someone help me fix this!! I’m new at this, but I think one of the files need changed/replaced, but I don’t know which one…