Panic: Blog Admin panel inaccessible after changing WordPress address URL

My blog runs at runinfinity.com.

Its WordPress address (URL) and Blog address (URL) were both set at
runinfinity.com.

For just trying out something, I changed the WordPress address (URL) to
http://runinfinity.com/runinfinity
(intent was the change blog directory under public_html to something different directly, rather that right there).

After doing this, I am totally stuck. Blog display has become weird (probably like when there was no theme), and I cannot login to Admin Panel too for the blog. Now when I write:
http://runinfinity.com/wp-admin
I am redirected to:
http://runinfinity.com/runinfinity/wp-login.php?redirect_to=http%3A%2F%2Fwww.runinfinity.com%2Fwp-admin%2F

and this just displays my blog as a 404 error.

I also tried using
http://runinfinity.com/runinfinity/wp-admin
same thing, a 404.

Please help, am really stuck on this. I know I should not have tried this, but what can I do now.