wordpress load error

I am new to wordpress. I was successfully able to install wordpress on to my site. Since it was taking too much time to load so I tried installing this plugin called “w3-total-cache”, but it didn’t install correctly and is not working anymore.

I am not worried about the plugin, but what I am worried about is that during the installation of the plugin I changed the “Permissions” of all files within the folder “wp-content” to “777” as it was told in the guide and when the plugin installation failed I changed it again to “644”. But now when I open my wordpress login page it shows the following errors :-

Warning: require() [function.require]: Unable to access /www/mydomain/org/htdocs/wordpress/wp-includes/load.php in /www/mydomain.org/mydomain/htdocs/wordpress/wp-settings.php on line 21

Warning: require(/www/mydomain/org/htdocs/wordpress/wp-includes/load.php) [function.require]: failed to open stream: Permission denied in /www/mydomain.org/mydomain/htdocs/wordpress/wp-settings.php on line 21

Fatal error: require() [function.require]: Failed opening required ‘/www/mydomain/org/htdocs/wordpress/wp-includes/load.php’ (include_path=’.:/usr/lib/php’) in /www/mydomain.org/mydomain/htdocs/wordpress/wp-settings.php on line 21

What should I do ?
Any suggestions ?