There are times when there is problem in your theme that you have installed in your site and your site goes blank or white. In this situation you will not be able to log in to the WordPress Admin area. In this situation you can simply use the database to choose the default theme and solve the problem.
Step 1: It is highly recommended to backup your database before your work on any database command.
Step 2: Login to your database using phpMyAdmin (or whatever), Click on your database and click on the SQL tab and enter the following SQL query:
UPDATE wp_options SET option_value='default' WHERE option_name='template' OR option_name='stylesheet' LIMIT 2;
This query will change the wordpress theme to default.