Why am I receiving a blank page after changing my phpBB theme?

As the exact cause of the issue cannot be said for sure, the best course of action is to change the template to the default one. To reset your phpBB2 template, there are two MySQL queries which you should run from your phpMyAdmin. Go to your Site Tools > Site > MySQL > phpMyAdmin. Choose the phpBB database from the menu on the left, click on SQL and paste the following queries in the text field:

UPDATE phpbb_config SET config_value = '1' WHERE config_name = 'default_style';
UPDATE phpbb_config SET config_value = '1' WHERE config_name = 'override_user_style';

Click on Go and this will submit the query. Clear your browser’s cache and open your forum.