How will you fix phpbb:critical error?
If you get phpBB: Critical Error, then in that case there is a chance the database is broken.
You have to run following sql queries either on mysql prompt or phpmyadmin :
UPDATE phpbb_config SET config_value = ‘ID_NUMBER_HERE’ WHERE config_name = ‘default_style’;
UPDATE phpbb_config SET config_value = ’1′ WHERE config_name = ‘override_user_style’;
If not , then you can refer :