Customizer Not Working or Showing a White Screen
Depending on your hosting (e.g. shared hosting, VPS, etc) and how big your database is, you might experience an issue when trying to access the Customizer, most notably where you only see blank page or long loading time. This is usually due to the Customizer timing out because of a memory limit in your hosting server. The first thing you can do to fix this is to enter the increase the memory limit of your hosting server via wp-config.php file. you just need to add this code there :
define( 'WP_MEMORY_LIMIT', '256M' ); define( 'WP_MAX_MEMORY_LIMIT', '512M' );
unless your hosting provider has a limit on these parameters, your customizer will working well now.
If your Customizer is still failing to load, the next step would be to open up a support ticket with your hosting provider and ask them to increase your PHP memory limit to a higher setting (preferably 512M)