This is also an experience I had with wordpress.
I have installed wordpress into a separate location of my hosting service (its means I didn't use the default location to install wordpress and installed them into a new folder called home), and I changed the "WordPress Address (URL)" under general settings into my web URL by misunderstanding the instruction given by wordpress, after changed this setting I couldn't access to wordpress site even it didn't allow me to access admin page, how ever finally I could resolve it using following method
01. Log into your MYSQL database using Phpmyadmin or any other methods provided by hosting service
02. Find your database belongs to Wordpress installation.
03. Open it and trace for the table called "wp_options" and find the field " WordPress Address (URL)" and change it into previous URL (http://yoursite.com/wordpress)
04. Then save your settings and check your site back, it should work fine!