Changing Site URL

a) I installed wp on subdirectory “mydomain” at domain http://mydomain.com.
b) I could access the dashboard for the site through the link http://mydomain.com/mydomain/wp-admin, after login.
c) My webpage opened up when I type the link http://mydomain.com/mydomain on the browser.
d) I recently fiddled with the settings on the “Settings->General” menu at “Home URL” and “Site URL” options in the dashboard and I am unable to access the login window for the site. What should I do now to restore.
e) What should I do so that I am able to access the site when I type http://www.mydomain.com on a browser window.

Shall be grateful if anybody could bail me out of this situation.

Thanks and Regards.