When disaster strikes your WordPress install, you have to act fast. Each time I move over an install or change a directory name for a client, the white screen of death makes an appearance. I can be related to a plugin, so first thing is to rename the plugin directory. Hard coding the links, deleting the catche (sometimes with template problems on multi-sites)
Taken from the wordpress codex:
WordPress stores two addresses inside the database. These determine where your blog files are, and where the main index is. On a normal install, these addresses are the same.
There are two occasions where you will need to access the database to alter one or both of these settings:
If you have tried to alter the Blog URL or WordPress URL in Settings, and an error has occurred.
If you have moved to a domain/subdomain with a different name.
Edit wp-config.php
It should be possible to fix the site URL using a new feature -
Add these two lines to your wp-config.php, where “example.com” is the NEW location of your site.
define('WP_HOME','http://example.com');
define('WP_SITEURL','http://example.com');
This is not necessarily the best fix, it’s just hardcoding the values into the site itself. You won’t be able to edit them on the General settings page anymore when using this method.
Thanks filosofo for the fix, implemented in 2.2.
Edit functions.php
If you have access to the site via FTP, then this method will help you quickly get a site back up and running, if you changed those values incorrectly.
1. FTP to the site, and get a copy of the active theme’s functions.php file. You’re going to edit it in a simple text editor (like notepad) and upload it back to the site.
2. Add these two lines to the file, immediately after the initial “
update_option('siteurl','http://example.com/blog');
update_option('home','http://example.com/blog');
Use your own URL instead of example.com, obviously.
3. Upload the file back to your site, in the same location. FileZilla offers a handy “edit file” function to do all of the above rapidly; if you can use that, do so.
4. Load the login or admin page a couple of times. The site should come back up.
5. Repeat the above steps, but remove those lines. IMPORTANT: Do NOT leave those lines in there. Remove them immediately after the site is up and running again.
If there is no functions.php file in the theme: Create a new text file called “functions.php”. Edit it with notepad, and add this text to it, using your own URL instead of example.com:
update_option('siteurl','http://example.com/blog');
update_option('home','http://example.com/blog');
Upload that to your theme directory, then proceed as stated above. Remove the file afterwards.
Read more here: wordpress codex: