Are you getting the whitescreen on blog itself, just the admin section, or both?
There are a few things you can try. First, make sure that safe_mode is off and memory_limit is set high enough in the php.ini file; 12-16MB should do it.
Tail the error log to see if there is any relevant information. If you are on a Linux server, run this command:
tail -fn0 <path to error log>
Then visit the page. This will output error messages in real-time.
If this doesn’t do it, it’s probably an issue with the theme. To change the theme back to the default, you can run one of the following queries on the MySQL database:
UPDATE wp_options SET option_value='default' WHERE option_name='template' OR option_name='stylesheet' LIMIT 2;
-OR-
UDPATE wp_options SET option_value='WordPress Default' WHERE option_name='current_theme' LIMIT 1;
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