can't login: edited WordPress address (URL) by mistake

I’m in big trouble!

I developed a renewal site at example.com/site and it is ready to go live at example.com. I thought I followed this Codex instruction to make the new site appear at example.com.
http://codex.wordpress.org/Giving_WordPress_Its_Own_Directory

But I must have mistakenly changed WordPress address (URL) to example.com (but that’s not where the files are). After I took care of the index.php and .htaccess, the new site did not appear at example.com

So I went back to the General settings page (that I kept it open) and chanced WordPress address (URL) back to example.com/site, and changed Site address (URL) to example.com. When I hit “save”, the admin page could not render. I think it’s because the previous wrong setting is still there.

Now I can’t get to my login page anymore. What can I do? Please someone help.